Improve leading calculation for consequent lines with different heights

Now layout behaves more like a line-stacking algorithm than a leading-only algorithm. Leading is applied partially as the top spacing before the line and partially as the bottom spacing instead of just being top spacing as before. For consequent lines with different heights this makes a lot of difference in a sense that lines will not overlap

DEVSIX-1053

Autoported commit.
Original commit hash: [484fa88]

Author: Snipx

itext.tests/itext.layout.tests/itext/layout/CollapsingMarginsTest.cs

@@ -193,7 +193,7 @@ public virtual void CollapsingMarginsTest04() {
             p.Add("When a man hath no freedom to fight for at home,\n" + "    Let him combat for that of his neighbours;\n"
                  + "Let him think of the glories of Greece and of Rome,\n" + "    And get knocked on the head for his labours.\n"
                  + "To do good to Mankind is the chivalrous plan,\n");
-            p.Add(new Text("small text").SetFontSize(6));
+            p.Add(new Text("small text").SetFontSize(5.1f));
             p.Add("\nAnd is always as nobly requited;\n" + "Then battle for Freedom wherever you can,\n" + "And, if not shot or hanged, you'll get knighted."
                 );
             Div div1 = new Div();

itext.tests/itext.layout.tests/resources/itext/layout/AutoTaggingTest/cmp_tableTest01.pdf

@@ -691,6 +691,14 @@ protected internal override MinMaxWidth GetMinMaxWidth(float availableWidth) {
             return result.GetNotNullMinMaxWidth(availableWidth);
         }
 
+        internal virtual float GetTopLeadingIndent(Leading leading) {
+            return (GetLeadingValue(leading) - occupiedArea.GetBBox().GetHeight()) / 2;
+        }
+
+        internal virtual float GetBottomLeadingIndent(Leading leading) {
+            return (GetLeadingValue(leading) - occupiedArea.GetBBox().GetHeight()) / 2;
+        }
+
         private LineRenderer[] SplitNotFittingFloat(int childPos, LayoutResult childResult) {
             LineRenderer[] split = Split();
             split[0].childRenderers.AddAll(childRenderers.SubList(0, childPos));

itext.tests/itext.layout.tests/resources/itext/layout/CollapsingMarginsTest/cmp_collapsingMarginsTest04.pdf

@@ -157,9 +157,7 @@ public override LayoutResult Layout(LayoutContext layoutContext) {
             }
             float lastYLine = layoutBox.GetY() + layoutBox.GetHeight();
             Leading leading = this.GetProperty<Leading>(Property.LEADING);
-            float leadingValue = 0;
-            float lastLineHeight = 0;
-            float lastLineLeading = 0;
+            float lastLineBottomLeadingIndent = 0;
             if (marginsCollapsingEnabled && childRenderers.Count > 0) {
                 // passing null is sufficient to notify that there is a kid, however we don't care about it and it's margins
                 marginsCollapseHandler.StartChildMarginsHandling(null, layoutBox);
@@ -222,25 +220,30 @@ public override LayoutResult Layout(LayoutContext layoutContext) {
                         }
                     }
                 }
-                leadingValue = processedRenderer != null && leading != null ? processedRenderer.GetLeadingValue(leading) : 
-                    0;
-                if (processedRenderer != null && processedRenderer.ContainsImage()) {
-                    leadingValue -= previousDescent;
-                }
                 bool lineHasContent = processedRenderer != null && processedRenderer.GetOccupiedArea().GetBBox().GetHeight
                     () > 0;
                 // could be false if e.g. line contains only floats
                 bool doesNotFit = processedRenderer == null;
                 float deltaY = 0;
                 if (!doesNotFit) {
                     if (lineHasContent) {
-                        lastLineLeading = leadingValue;
-                        lastLineHeight = processedRenderer.GetOccupiedArea().GetBBox().GetHeight();
-                        deltaY = lastYLine - leadingValue - processedRenderer.GetYLine();
+                        float indentFromLastLine = previousDescent - lastLineBottomLeadingIndent - (leading != null ? processedRenderer
+                            .GetTopLeadingIndent(leading) : 0) - processedRenderer.GetMaxAscent();
+                        // TODO this is a workaround. To be refactored
+                        if (processedRenderer != null && processedRenderer.ContainsImage()) {
+                            indentFromLastLine += previousDescent;
+                        }
+                        deltaY = lastYLine + indentFromLastLine - processedRenderer.GetYLine();
+                        lastLineBottomLeadingIndent = leading != null ? processedRenderer.GetBottomLeadingIndent(leading) : 0;
+                        // TODO this is a workaround. To be refactored
+                        if (lastLineBottomLeadingIndent < 0 && processedRenderer.ContainsImage()) {
+                            lastLineBottomLeadingIndent = 0;
+                        }
                     }
                     // for the first and last line in a paragraph, leading is smaller
                     if (firstLineInBox) {
-                        deltaY = -(leadingValue - lastLineHeight) / 2;
+                        deltaY = processedRenderer != null && leading != null ? -processedRenderer.GetTopLeadingIndent(leading) : 
+                            0;
                     }
                     doesNotFit = leading != null && processedRenderer.GetOccupiedArea().GetBBox().GetY() + deltaY < layoutBox.
                         GetY();
@@ -362,8 +365,7 @@ public override LayoutResult Layout(LayoutContext layoutContext) {
             if (FloatingHelper.IsRendererFloating(this, floatPropertyValue)) {
                 FloatingHelper.IncludeChildFloatsInOccupiedArea(floatRendererAreas, this);
             }
-            float moveDown = Math.Min((lastLineLeading - lastLineHeight) / 2, occupiedArea.GetBBox().GetY() - layoutBox
-                .GetY());
+            float moveDown = Math.Min(lastLineBottomLeadingIndent, occupiedArea.GetBBox().GetY() - layoutBox.GetY());
             occupiedArea.GetBBox().MoveDown(moveDown);
             occupiedArea.GetBBox().SetHeight(occupiedArea.GetBBox().GetHeight() + moveDown);
             IRenderer overflowRenderer = null;

itext.tests/itext.layout.tests/resources/itext/layout/OverflowTest/cmp_textOverflowTest02.pdf

@@ -1 +1 @@
-898612dbe6c7a27077e10b543b975bebd887eefb
+484fa88ec7cc665e4b684a76472ba230ac66658e