[Impeller] Fix convex triangulation winding bug for multi-contour paths. (#51198)

An extra triangle needs to be inserted after bridging the triangle strip to a new contour, otherwise the triangle winding ends up getting inverted for each consecutive contour.

Before:

<img width="685" alt="image" src="https://github.com/flutter/engine/assets/919017/cef666c1-c188-4c83-ae93-fca117b0bc7f">

After:

<img width="685" alt="image" src="https://github.com/flutter/engine/assets/919017/0cf5e994-5366-4e73-8f9c-4fec1128b303">

Author: bdero

impeller/aiks/aiks_path_unittests.cc

@@ -398,5 +398,37 @@ TEST_P(AiksTest, CanRenderClips) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, CanRenderOverlappingMultiContourPath) {
+  Canvas canvas;
+
+  Paint paint;
+  paint.color = Color::Red();
+
+  PathBuilder::RoundingRadii radii;
+  radii.top_left = {50, 50};
+  radii.top_right = {50, 50};
+  radii.bottom_right = {50, 50};
+  radii.bottom_left = {50, 50};
+
+  const Scalar kTriangleHeight = 100;
+  canvas.Translate(Vector2(200, 200));
+  // Form a path similar to the Material drop slider value indicator.
+  auto path =
+      PathBuilder{}
+          .MoveTo({0, kTriangleHeight})
+          .LineTo({-kTriangleHeight / 2.0f, 0})
+          .LineTo({kTriangleHeight / 2.0f, 0})
+          .Close()
+          .AddRoundedRect(
+              Rect::MakeXYWH(-kTriangleHeight / 2.0f, -kTriangleHeight / 2.0f,
+                             kTriangleHeight, kTriangleHeight),
+              radii)
+          .TakePath();
+
+  canvas.DrawPath(path, paint);
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller

impeller/tessellator/tessellator.cc

@@ -205,6 +205,7 @@ std::vector<Point> Tessellator::TessellateConvex(const Path& path,
       output.emplace_back(output.back());
       output.emplace_back(first_point);
       output.emplace_back(first_point);
+      output.emplace_back(first_point);
     } else {
       output.emplace_back(first_point);
     }

impeller/tessellator/tessellator_unittests.cc

@@ -126,8 +126,8 @@ TEST(TessellatorTest, TessellateConvex) {
                                   1.0);
 
     std::vector<Point> expected = {{0, 0},   {10, 0},  {0, 10},  {10, 10},
-                                   {10, 10}, {20, 20}, {20, 20}, {30, 20},
-                                   {20, 30}, {30, 30}};
+                                   {10, 10}, {20, 20}, {20, 20}, {20, 20},
+                                   {30, 20}, {20, 30}, {30, 30}};
     EXPECT_EQ(pts, expected);
   }
 }

testing/impeller_golden_tests_output.txt

@@ -284,6 +284,9 @@ impeller_Play_AiksTest_CanRenderNestedClips_Vulkan.png
 impeller_Play_AiksTest_CanRenderOffscreenCheckerboard_Metal.png
 impeller_Play_AiksTest_CanRenderOffscreenCheckerboard_OpenGLES.png
 impeller_Play_AiksTest_CanRenderOffscreenCheckerboard_Vulkan.png
+impeller_Play_AiksTest_CanRenderOverlappingMultiContourPath_Metal.png
+impeller_Play_AiksTest_CanRenderOverlappingMultiContourPath_OpenGLES.png
+impeller_Play_AiksTest_CanRenderOverlappingMultiContourPath_Vulkan.png
 impeller_Play_AiksTest_CanRenderRadialGradientManyColors_Metal.png
 impeller_Play_AiksTest_CanRenderRadialGradientManyColors_OpenGLES.png
 impeller_Play_AiksTest_CanRenderRadialGradientManyColors_Vulkan.png