fully support pinned position

Author: fbchen

example/pinned_position.dart

@@ -31,6 +31,7 @@ class PinnedPositionExample extends StatelessWidget {
               anchor,
               PinnedPos(0, PinnedType.absolute, 0.5, PinnedType.percent),
               PinnedPos(0.5, PinnedType.percent, 0.5, PinnedType.percent),
+              rotateDegree: 45,
             ),
           )
         ],

lib/src/constraint_layout.dart

@@ -3777,8 +3777,29 @@ class _ConstraintRenderBox extends RenderBox
       if (!element.translateConstraint) {
         paintShift = element.translate;
       }
-      context.paintChild(
-          element.renderBox!, element.offset + offset + paintShift);
+
+      if (element.pinnedInfo != null) {
+        context.canvas.save();
+        context.canvas.translate(
+            element.offset.dx +
+                offset.dx +
+                paintShift.dx +
+                element.getMeasuredWidth() / 2,
+            element.offset.dy +
+                offset.dy +
+                paintShift.dy +
+                element.getMeasuredHeight() / 2);
+        context.canvas
+            .rotate(pi + pi * (element.pinnedInfo!.rotateDegree / 180));
+        context.paintChild(
+            element.renderBox!,
+            Offset(-element.getMeasuredWidth() / 2,
+                -element.getMeasuredHeight() / 2));
+        context.canvas.restore();
+      } else {
+        context.paintChild(
+            element.renderBox!, element.offset + offset + paintShift);
+      }
 
       /// Draw child's click area
       assert(() {