Own main workers and narrow Apple render hooks

src/legacy_canvas_draw_merge_services.h

@@ -118,6 +118,16 @@ struct LegacyCanvasDrawMergeTemporaryCompositeExecution {
     std::function<void()> unbind_textures;
 };
 
+struct LegacyCanvasDrawMergeBackgroundSetupUniforms {
+    bool draw_merged = false;
+    bool use_blend = false;
+};
+
+struct LegacyCanvasDrawMergeBackgroundSetupExecution {
+    std::function<void()> disable_blend;
+    std::function<void(int)> draw_checkerboard_plane;
+};
+
 template <
     typename Setup,
     typename BindSamplers,
@@ -485,6 +495,21 @@ inline void execute_legacy_canvas_draw_merge_temporary_composite(
     execution.unbind_textures();
 }
 
+inline void execute_legacy_canvas_draw_merge_background_setup(
+    const LegacyCanvasDrawMergeBackgroundSetupUniforms& uniforms,
+    const LegacyCanvasDrawMergeBackgroundSetupExecution& execution)
+{
+    if (uniforms.draw_merged) {
+        execution.disable_blend();
+    }
+
+    if (uniforms.draw_merged || !uniforms.use_blend) {
+        for (int plane_index = 0; plane_index < 6; ++plane_index) {
+            execution.draw_checkerboard_plane(plane_index);
+        }
+    }
+}
+
 inline void execute_legacy_canvas_draw_merge_plane_setup(
     const LegacyCanvasDrawMergePlaneSetupUniforms& uniforms,
     const LegacyCanvasDrawMergePlaneSetupExecution& execution)