Extract Canvas draw_merge layer blend helper

src/legacy_canvas_draw_merge_services.h

@@ -64,6 +64,22 @@ struct LegacyCanvasDrawMergeShaderExecution {
     std::function<void(kShaderUniform, const glm::mat4&)> set_mat4;
 };
 
+struct LegacyCanvasDrawMergeLayerBlendUniforms {
+    LegacyCanvasDrawMergeTextureBlendUniforms shader;
+    bool copy_destination = false;
+};
+
+struct LegacyCanvasDrawMergeLayerBlendExecution {
+    std::function<void()> unbind_merge_framebuffer;
+    std::function<void()> bind_samplers;
+    std::function<void()> bind_merge_texture;
+    std::function<void()> bind_destination_texture;
+    std::function<void()> copy_destination_framebuffer;
+    std::function<void()> draw;
+    std::function<void()> unbind_destination_texture;
+    std::function<void()> unbind_merge_texture;
+};
+
 [[nodiscard]] inline LegacyCanvasDrawMergeShaderExecution legacy_shader_manager_draw_merge_execution() noexcept
 {
     return {
@@ -218,4 +234,27 @@ inline void setup_legacy_canvas_draw_merge_texture_colorize_shader(
     setup_legacy_canvas_draw_merge_texture_colorize_shader(uniforms, legacy_shader_manager_draw_merge_execution());
 }
 
+inline void execute_legacy_canvas_draw_merge_layer_blend(
+    const LegacyCanvasDrawMergeLayerBlendUniforms& uniforms,
+    const LegacyCanvasDrawMergeLayerBlendExecution& execution)
+{
+    execution.unbind_merge_framebuffer();
+    execution.bind_samplers();
+    setup_legacy_canvas_draw_merge_texture_blend_shader(uniforms.shader);
+    execution.bind_merge_texture();
+
+    if (uniforms.copy_destination) {
+        execution.bind_destination_texture();
+        execution.copy_destination_framebuffer();
+    }
+
+    execution.draw();
+
+    if (uniforms.copy_destination) {
+        execution.unbind_destination_texture();
+    }
+
+    execution.unbind_merge_texture();
+}
+
 } // namespace pp::panopainter