[Impeller] Ensure that reused textures are cleared before getting sampled by backdrop textures (#46084)

Fix for flutter/flutter#135053.

I'm going to try and optimize out this case later, since Wondrous is
inadvertently paying for a backdrop filter that contributes nothing to
the final image.

Author: bdero

impeller/aiks/aiks_unittests.cc

@@ -2264,6 +2264,30 @@ TEST_P(AiksTest, DrawPaintAbsorbsClears) {
   ASSERT_EQ(render_pass->GetCommands().size(), 0llu);
 }
 
+// This is important to enforce with texture reuse, since cached textures need
+// to be cleared before reuse.
+TEST_P(AiksTest,
+       ParentSaveLayerCreatesRenderPassWhenChildBackdropFilterIsPresent) {
+  Canvas canvas;
+  canvas.SaveLayer({}, std::nullopt, ImageFilter::MakeMatrix(Matrix(), {}));
+  canvas.DrawPaint({.color = Color::Red(), .blend_mode = BlendMode::kSource});
+  canvas.DrawPaint({.color = Color::CornflowerBlue().WithAlpha(0.75),
+                    .blend_mode = BlendMode::kSourceOver});
+  canvas.Restore();
+
+  Picture picture = canvas.EndRecordingAsPicture();
+
+  std::shared_ptr<ContextSpy> spy = ContextSpy::Make();
+  std::shared_ptr<Context> real_context = GetContext();
+  std::shared_ptr<ContextMock> mock_context = spy->MakeContext(real_context);
+  AiksContext renderer(mock_context, nullptr);
+  std::shared_ptr<Image> image = picture.ToImage(renderer, {300, 300});
+
+  ASSERT_EQ(spy->render_passes_.size(), 3llu);
+  std::shared_ptr<RenderPass> render_pass = spy->render_passes_[0];
+  ASSERT_EQ(render_pass->GetCommands().size(), 0llu);
+}
+
 TEST_P(AiksTest, DrawRectAbsorbsClears) {
   Canvas canvas;
   canvas.DrawRect({0, 0, 300, 300},

impeller/entity/entity_pass.cc

@@ -534,6 +534,14 @@ EntityPass::EntityResult EntityPass::GetEntityForElement(
           proc(FilterInput::Make(std::move(texture)),
                subpass->xformation_.Basis(), Entity::RenderingMode::kSubpass);
 
+      // If the very first thing we render in this EntityPass is a subpass that
+      // happens to have a backdrop filter, than that backdrop filter will end
+      // may wind up sampling from the raw, uncleared texture that came straight
+      // out of the texture cache. By calling `pass_context.GetRenderPass` here,
+      // we force the texture to pass through at least one RenderPass with the
+      // correct clear configuration before any sampling occurs.
+      pass_context.GetRenderPass(pass_depth);
+
       // 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
       // rendering the subpass.