[Impeller] Only strip the translation from backdrop filter effect transforms. (flutter#44029)

Resolves flutter/flutter#131305.

Author: bdero

impeller/aiks/aiks_unittests.cc

@@ -2930,6 +2930,38 @@ TEST_P(AiksTest, DrawPictureWithText) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, MatrixSaveLayerFilter) {
+  Canvas canvas;
+  canvas.DrawPaint({.color = Color::Black()});
+  canvas.SaveLayer({}, std::nullopt);
+  {
+    canvas.DrawCircle(Point(200, 200), 100,
+                      {.color = Color::Green().WithAlpha(0.5),
+                       .blend_mode = BlendMode::kPlus});
+    // Should render a second circle, centered on the bottom-right-most edge of
+    // the circle.
+    canvas.SaveLayer({.image_filter =
+                          [](const FilterInput::Ref& input,
+                             const Matrix& effect_transform, bool is_subpass) {
+                            Matrix matrix =
+                                Matrix::MakeTranslation(
+                                    Vector2(1, 1) * (100 + 100 * k1OverSqrt2)) *
+                                Matrix::MakeScale(Vector2(1, 1) * 0.2) *
+                                Matrix::MakeTranslation(Vector2(-100, -100));
+                            return FilterContents::MakeMatrixFilter(
+                                input, matrix, {}, Matrix(), true);
+                          }},
+                     std::nullopt);
+    canvas.DrawCircle(Point(200, 200), 100,
+                      {.color = Color::Green().WithAlpha(0.5),
+                       .blend_mode = BlendMode::kPlus});
+    canvas.Restore();
+  }
+  canvas.Restore();
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 TEST_P(AiksTest, MatrixBackdropFilter) {
   Canvas canvas;
   canvas.DrawPaint({.color = Color::Black()});

impeller/entity/contents/filters/matrix_filter_contents.cc

@@ -49,8 +49,7 @@ std::optional<Entity> MatrixFilterContents::RenderFilter(
   // mentioned above). And so we sneak the subpass's captured CTM in through the
   // effect transform.
 
-  auto transform = is_subpass_ ? effect_transform.Basis()
-                               : entity.GetTransformation().Basis();
+  auto transform = is_subpass_ ? effect_transform : entity.GetTransformation();
   snapshot->transform = transform *           //
                         matrix_ *             //
                         transform.Invert() *  //

impeller/entity/entity_pass.cc

@@ -470,9 +470,9 @@ EntityPass::EntityResult EntityPass::GetEntityForElement(
       auto texture = pass_context.GetTexture();
       // Render the backdrop texture before any of the pass elements.
       const auto& proc = subpass->backdrop_filter_proc_;
-      subpass_backdrop_filter_contents =
-          proc(FilterInput::Make(std::move(texture)), subpass->xformation_,
-               /*is_subpass*/ true);
+      subpass_backdrop_filter_contents = proc(
+          FilterInput::Make(std::move(texture)), subpass->xformation_.Basis(),
+          /*is_subpass*/ true);
 
       // The subpass will need to read from the current pass texture when
       // rendering the backdrop, so if there's an active pass, end it prior to