Extract Canvas draw_merge final plane composite helper

src/legacy_canvas_draw_merge_services.h

@@ -80,6 +80,22 @@ struct LegacyCanvasDrawMergeLayerBlendExecution {
     std::function<void()> unbind_merge_texture;
 };
 
+struct LegacyCanvasDrawMergeFinalPlaneCompositeUniforms {
+    LegacyCanvasDrawMergeCheckerboardUniforms checkerboard;
+    LegacyCanvasDrawMergeTextureUniforms texture;
+    bool draw_checkerboard = false;
+};
+
+struct LegacyCanvasDrawMergeFinalPlaneCompositeExecution {
+    std::function<void()> bind_merged_texture_copy_target;
+    std::function<void()> copy_merged_framebuffer;
+    std::function<void()> enable_blend;
+    std::function<void()> draw;
+    std::function<void()> bind_sampler;
+    std::function<void()> bind_merged_texture;
+    std::function<void()> unbind_merged_texture;
+};
+
 [[nodiscard]] inline LegacyCanvasDrawMergeShaderExecution legacy_shader_manager_draw_merge_execution() noexcept
 {
     return {
@@ -257,4 +273,25 @@ inline void execute_legacy_canvas_draw_merge_layer_blend(
     execution.unbind_merge_texture();
 }
 
+inline void execute_legacy_canvas_draw_merge_final_plane_composite(
+    const LegacyCanvasDrawMergeFinalPlaneCompositeUniforms& uniforms,
+    const LegacyCanvasDrawMergeFinalPlaneCompositeExecution& execution)
+{
+    execution.bind_merged_texture_copy_target();
+    execution.copy_merged_framebuffer();
+
+    execution.enable_blend();
+
+    if (uniforms.draw_checkerboard) {
+        setup_legacy_canvas_draw_merge_checkerboard_shader(uniforms.checkerboard);
+        execution.draw();
+    }
+
+    execution.bind_sampler();
+    execution.bind_merged_texture();
+    setup_legacy_canvas_draw_merge_texture_shader(uniforms.texture);
+    execution.draw();
+    execution.unbind_merged_texture();
+}
+
 } // namespace pp::panopainter