Route NodeCanvas blend setup through helper

src/legacy_canvas_draw_merge_services.h

@@ -16,9 +16,19 @@ struct LegacyCanvasDrawMergeTextureUniforms {
     int texture_slot = 0;
 };
 
+struct LegacyCanvasDrawMergeTextureBlendUniforms {
+    glm::mat4 mvp { 1.0f };
+    int texture_slot = 0;
+    int destination_texture_slot = 2;
+    bool use_destination_texture = false;
+    int blend_mode = 0;
+    float alpha = 1.0f;
+};
+
 struct LegacyCanvasDrawMergeShaderExecution {
     std::function<void(kShader)> use_shader;
     std::function<void(kShaderUniform, int)> set_int;
+    std::function<void(kShaderUniform, float)> set_float;
     std::function<void(kShaderUniform, const glm::mat4&)> set_mat4;
 };
 
@@ -27,6 +37,7 @@ struct LegacyCanvasDrawMergeShaderExecution {
     return {
         .use_shader = [](kShader shader) { ShaderManager::use(shader); },
         .set_int = [](kShaderUniform uniform, int value) { ShaderManager::u_int(uniform, value); },
+        .set_float = [](kShaderUniform uniform, float value) { ShaderManager::u_float(uniform, value); },
         .set_mat4 = [](kShaderUniform uniform, const glm::mat4& value) { ShaderManager::u_mat4(uniform, value); },
     };
 }
@@ -61,4 +72,24 @@ inline void setup_legacy_canvas_draw_merge_texture_shader(
     setup_legacy_canvas_draw_merge_texture_shader(uniforms, legacy_shader_manager_draw_merge_execution());
 }
 
+inline void setup_legacy_canvas_draw_merge_texture_blend_shader(
+    const LegacyCanvasDrawMergeTextureBlendUniforms& uniforms,
+    const LegacyCanvasDrawMergeShaderExecution& execution) noexcept
+{
+    execution.use_shader(kShader::TextureBlend);
+    execution.set_int(kShaderUniform::Tex, uniforms.texture_slot);
+    if (uniforms.use_destination_texture) {
+        execution.set_int(kShaderUniform::TexBG, uniforms.destination_texture_slot);
+    }
+    execution.set_int(kShaderUniform::BlendMode, uniforms.blend_mode);
+    execution.set_float(kShaderUniform::Alpha, uniforms.alpha);
+    execution.set_mat4(kShaderUniform::MVP, uniforms.mvp);
+}
+
+inline void setup_legacy_canvas_draw_merge_texture_blend_shader(
+    const LegacyCanvasDrawMergeTextureBlendUniforms& uniforms)
+{
+    setup_legacy_canvas_draw_merge_texture_blend_shader(uniforms, legacy_shader_manager_draw_merge_execution());
+}
+
 } // namespace pp::panopainter