[Impeller] Fix DrawPicture. (flutter#43446)

Prior to this patch, `DrawPicture` was untested and a no-op.

We needed a new routine to absorb Elements of a cloned pass into another pass, since appending a `Picture` as a subpass would be incorrect behavior for drawing a picture (since subpasses start with a blank image, which gets drawn to and then composited with the parent pass via a separate blending operation).

impeller/aiks/aiks_unittests.cc

@@ -2837,5 +2837,19 @@ TEST_P(AiksTest, TextForegroundShaderWithTransform) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, CanCanvasDrawPicture) {
+  Canvas subcanvas;
+  subcanvas.DrawRect(Rect::MakeLTRB(-100, -50, 100, 50),
+                     {.color = Color::CornflowerBlue()});
+  auto picture = subcanvas.EndRecordingAsPicture();
+
+  Canvas canvas;
+  canvas.Translate({200, 200});
+  canvas.Rotate(Radians(kPi / 4));
+  canvas.DrawPicture(picture);
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller

impeller/aiks/canvas.cc

@@ -401,10 +401,14 @@ void Canvas::DrawPoints(std::vector<Point> points,
   GetCurrentPass().AddEntity(entity);
 }
 
-void Canvas::DrawPicture(Picture picture) {
+void Canvas::DrawPicture(const Picture& picture) {
   if (!picture.pass) {
     return;
   }
+
+  auto save_count = GetSaveCount();
+  Save();
+
   // Clone the base pass and account for the CTM updates.
   auto pass = picture.pass->Clone();
   pass->IterateAllEntities([&](auto& entity) -> bool {
@@ -413,7 +417,9 @@ void Canvas::DrawPicture(Picture picture) {
                              entity.GetTransformation());
     return true;
   });
-  return;
+  GetCurrentPass().AddSubpassInline(std::move(pass));
+
+  RestoreToCount(save_count);
 }
 
 void Canvas::DrawImage(const std::shared_ptr<Image>& image,

impeller/aiks/canvas.h

@@ -137,7 +137,7 @@ class Canvas {
       Scalar corner_radius,
       Entity::ClipOperation clip_op = Entity::ClipOperation::kIntersect);
 
-  void DrawPicture(Picture picture);
+  void DrawPicture(const Picture& picture);
 
   void DrawTextFrame(const TextFrame& text_frame,
                      Point position,

impeller/entity/entity_pass.cc

@@ -167,6 +167,22 @@ EntityPass* EntityPass::AddSubpass(std::unique_ptr<EntityPass> pass) {
   return subpass_pointer;
 }
 
+void EntityPass::AddSubpassInline(std::unique_ptr<EntityPass> pass) {
+  if (!pass) {
+    return;
+  }
+  FML_DCHECK(pass->superpass_ == nullptr);
+
+  elements_.insert(elements_.end(),
+                   std::make_move_iterator(pass->elements_.begin()),
+                   std::make_move_iterator(pass->elements_.end()));
+
+  backdrop_filter_reads_from_pass_texture_ +=
+      pass->backdrop_filter_reads_from_pass_texture_;
+  advanced_blend_reads_from_pass_texture_ +=
+      pass->advanced_blend_reads_from_pass_texture_;
+}
+
 static RenderTarget::AttachmentConfig GetDefaultStencilConfig(bool readable) {
   return RenderTarget::AttachmentConfig{
       .storage_mode = readable ? StorageMode::kDevicePrivate

impeller/entity/entity_pass.h

@@ -62,13 +62,21 @@ class EntityPass {
 
   void SetElements(std::vector<Element> elements);
 
+  /// @brief  Appends a given pass as a subpass.
   EntityPass* AddSubpass(std::unique_ptr<EntityPass> pass);
 
+  /// @brief  Merges a given pass into this pass. Useful for drawing
+  ///         pre-recorded pictures that don't require rendering into a separate
+  ///         subpass.
+  void AddSubpassInline(std::unique_ptr<EntityPass> pass);
+
   EntityPass* GetSuperpass() const;
 
   bool Render(ContentContext& renderer,
               const RenderTarget& render_target) const;
 
+  /// @brief  Iterate all entities in this pass, recursively including entities
+  ///         of child passes. The iteration order is depth-first.
   void IterateAllEntities(const std::function<bool(Entity&)>& iterator);
 
   /// @brief  Iterate entities in this pass up until the first subpass is found.