Tweaked TextContents math to avoid floating point errors (#162480)

This gets rid of artifacts in characters.

## before

![before_445](https://github.com/user-attachments/assets/c66d3d22-baaf-4d66-bff3-3ad5b4d5747c)

## after

![after_445](https://github.com/user-attachments/assets/0e23a593-b68a-4334-a82e-4f7dc1ecdc17)


## Pre-launch Checklist

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there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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engine/src/flutter/impeller/display_list/aiks_dl_text_unittests.cc

@@ -155,6 +155,28 @@ TEST_P(AiksTest, CanRenderTextFrameWithHalfScaling) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+// This is a test that looks for glyph artifacts we've see.
+TEST_P(AiksTest, ScaledK) {
+  DisplayListBuilder builder;
+  DlPaint paint;
+  paint.setColor(DlColor::ARGB(1, 0.1, 0.1, 0.1));
+  builder.DrawPaint(paint);
+  for (int i = 0; i < 6; ++i) {
+    builder.Save();
+    builder.Translate(300 * i, 0);
+    Scalar scale = 0.445 - (i / 1000.f);
+    builder.Scale(scale, scale);
+    RenderTextInCanvasSkia(
+        GetContext(), builder, "k", "Roboto-Regular.ttf",
+        TextRenderOptions{.font_size = 600, .position = DlPoint(10, 500)});
+    RenderTextInCanvasSkia(
+        GetContext(), builder, "k", "Roboto-Regular.ttf",
+        TextRenderOptions{.font_size = 300, .position = DlPoint(10, 800)});
+    builder.Restore();
+  }
+  ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
+}
+
 TEST_P(AiksTest, CanRenderTextFrameWithFractionScaling) {
   Scalar fine_scale = 0.f;
   bool is_subpixel = false;

engine/src/flutter/impeller/entity/contents/text_contents.cc

@@ -187,7 +187,9 @@ void TextContents::ComputeVertexData(
         Point position;
         if (is_translation_scale) {
           position = (screen_glyph_position +
-                      (basis_transform * point * scaled_bounds.GetSize()))
+                      ((basis_transform.m[0] < 0 ? Matrix::MakeScale({-1, 1, 1})
+                                                 : Matrix()) *
+                       point * glyph_bounds.GetSize()))
                          .Round();
         } else {
           position = entity_transform *

engine/src/flutter/testing/impeller_golden_tests_output.txt

@@ -880,6 +880,9 @@ impeller_Play_AiksTest_SaveLayerDrawsBehindSubsequentEntities_Vulkan.png
 impeller_Play_AiksTest_SaveLayerFiltersScaleWithTransform_Metal.png
 impeller_Play_AiksTest_SaveLayerFiltersScaleWithTransform_OpenGLES.png
 impeller_Play_AiksTest_SaveLayerFiltersScaleWithTransform_Vulkan.png
+impeller_Play_AiksTest_ScaledK_Metal.png
+impeller_Play_AiksTest_ScaledK_OpenGLES.png
+impeller_Play_AiksTest_ScaledK_Vulkan.png
 impeller_Play_AiksTest_SetContentsWithRegion_Metal.png
 impeller_Play_AiksTest_SetContentsWithRegion_OpenGLES.png
 impeller_Play_AiksTest_SetContentsWithRegion_Vulkan.png