[Impeller] Fix vertex allocation counts for flat curves (flutter#170194)

The code that estimated the vertex counts for filling paths was not
taking into account that the minimum number of points allocated for a
curve segment is 1. The iteration estimates will return "don't bother to
subdivide the curve" cases as "0 subdivisions" and we estimated no
vertices would be generated. In fact, the rendering code would always
emit the final point regardless of the subdivision estimate. Other cases
that returned a value >0 were already accurate.

Author: flar

engine/src/flutter/impeller/tessellator/path_tessellator.cc

@@ -194,17 +194,21 @@ class StorageCounter : public SegmentReceiver {
   void RecordLine(Point p1, Point p2) override { point_count_++; }
 
   void RecordQuad(Point p1, Point cp, Point p2) override {
-    point_count_ += std::ceil(ComputeQuadradicSubdivisions(scale_, p1, cp, p2));
+    size_t count =  //
+        std::ceilf(ComputeQuadradicSubdivisions(scale_, p1, cp, p2));
+    point_count_ += std::max<size_t>(count, 1);
   }
 
   void RecordConic(Point p1, Point cp, Point p2, Scalar weight) override {
-    point_count_ +=
-        std::ceil(ComputeConicSubdivisions(scale_, p1, cp, p2, weight));
+    size_t count =  //
+        std::ceilf(ComputeConicSubdivisions(scale_, p1, cp, p2, weight));
+    point_count_ += std::max<size_t>(count, 1);
   }
 
   void RecordCubic(Point p1, Point cp1, Point cp2, Point p2) override {
-    point_count_ +=
-        std::ceil(ComputeCubicSubdivisions(scale_, p1, cp1, cp2, p2));
+    size_t count =  //
+        std::ceilf(ComputeCubicSubdivisions(scale_, p1, cp1, cp2, p2));
+    point_count_ += std::max<size_t>(count, 1);
   }
 
   void EndContour(Point origin, bool with_close) override {

engine/src/flutter/impeller/tessellator/path_tessellator_unittests.cc

@@ -544,5 +544,41 @@ TEST(PathTessellatorTest, ComplexPathTrailingMoveTo) {
   PathTessellator::PathToFilledVertices(path, mock_writer, 1.0f);
 }
 
+TEST(PathTessellatorTest, LinearQuadToPointCount) {
+  flutter::DlPathBuilder builder;
+  builder.MoveTo({316.3, 121.5});
+  builder.QuadraticCurveTo({316.4, 121.5}, {316.5, 121.5});
+  builder.Close();
+  auto path = builder.TakePath();
+
+  auto [points, contours] = PathTessellator::CountFillStorage(path, 2.0f);
+  EXPECT_EQ(points, 3u);
+  EXPECT_EQ(contours, 1u);
+}
+
+TEST(PathTessellatorTest, LinearConicToPointCount) {
+  flutter::DlPathBuilder builder;
+  builder.MoveTo({316.3, 121.5});
+  builder.ConicCurveTo({316.4, 121.5}, {316.5, 121.5}, 2.0f);
+  builder.Close();
+  auto path = builder.TakePath();
+
+  auto [points, contours] = PathTessellator::CountFillStorage(path, 2.0f);
+  EXPECT_EQ(points, 3u);
+  EXPECT_EQ(contours, 1u);
+}
+
+TEST(PathTessellatorTest, LinearCubicToPointCount) {
+  flutter::DlPathBuilder builder;
+  builder.MoveTo({316.3, 121.5});
+  builder.CubicCurveTo({316.4, 121.5}, {316.5, 121.5}, {316.6, 121.5});
+  builder.Close();
+  auto path = builder.TakePath();
+
+  auto [points, contours] = PathTessellator::CountFillStorage(path, 2.0f);
+  EXPECT_EQ(points, 3u);
+  EXPECT_EQ(contours, 1u);
+}
+
 }  // namespace testing
 }  // namespace impeller