Centralize canvas blend gate planning

src/paint_renderer/compositor.h

@@ -50,6 +50,30 @@ struct StrokeCompositePlan {
     bool requires_explicit_transition = false;
 };
 
+struct CanvasBlendGateRequest {
+    pp::renderer::Extent2D extent {};
+    std::span<const int> layer_blend_modes;
+    bool has_stroke_blend_mode = false;
+    int stroke_blend_mode = 0;
+    bool dual_brush_blend = false;
+    bool pattern_blend = false;
+};
+
+struct CanvasBlendGatePlan {
+    bool shader_blend = false;
+    bool complex_blend = false;
+    bool compatibility_fallback = false;
+    bool stroke_complex = false;
+    bool dual_brush_complex = false;
+    bool pattern_complex = false;
+    int first_complex_layer_index = -1;
+    StrokeCompositePath path = StrokeCompositePath::fixed_function_blend;
+    bool reads_destination_color = false;
+    bool requires_auxiliary_texture = false;
+    bool requires_texture_copy = false;
+    bool requires_render_target_blit = false;
+};
+
 [[nodiscard]] pp::foundation::Status composite_layer(
     std::span<pp::paint::Rgba> destination,
     pp::renderer::Extent2D extent,
@@ -65,6 +89,10 @@ struct StrokeCompositePlan {
     pp::renderer::RenderDeviceFeatures features,
     StrokeCompositeRequest request) noexcept;
 
+[[nodiscard]] pp::foundation::Result<CanvasBlendGatePlan> plan_canvas_blend_gate(
+    pp::renderer::RenderDeviceFeatures features,
+    CanvasBlendGateRequest request) noexcept;
+
 [[nodiscard]] const char* stroke_composite_path_name(StrokeCompositePath path) noexcept;
 
 }