Isolate draw merge background shader setup

src/legacy_canvas_draw_merge_services.h

@@ -0,0 +1,64 @@
+#pragma once
+
+#include "shader.h"
+
+#include <functional>
+
+namespace pp::panopainter {
+
+struct LegacyCanvasDrawMergeCheckerboardUniforms {
+    glm::mat4 mvp { 1.0f };
+    bool colorize = false;
+};
+
+struct LegacyCanvasDrawMergeTextureUniforms {
+    glm::mat4 mvp { 1.0f };
+    int texture_slot = 0;
+};
+
+struct LegacyCanvasDrawMergeShaderExecution {
+    std::function<void(kShader)> use_shader;
+    std::function<void(kShaderUniform, int)> set_int;
+    std::function<void(kShaderUniform, const glm::mat4&)> set_mat4;
+};
+
+[[nodiscard]] inline LegacyCanvasDrawMergeShaderExecution legacy_shader_manager_draw_merge_execution() noexcept
+{
+    return {
+        .use_shader = [](kShader shader) { ShaderManager::use(shader); },
+        .set_int = [](kShaderUniform uniform, int value) { ShaderManager::u_int(uniform, value); },
+        .set_mat4 = [](kShaderUniform uniform, const glm::mat4& value) { ShaderManager::u_mat4(uniform, value); },
+    };
+}
+
+inline void setup_legacy_canvas_draw_merge_checkerboard_shader(
+    const LegacyCanvasDrawMergeCheckerboardUniforms& uniforms,
+    const LegacyCanvasDrawMergeShaderExecution& execution) noexcept
+{
+    execution.use_shader(kShader::Checkerboard);
+    execution.set_int(kShaderUniform::Colorize, uniforms.colorize);
+    execution.set_mat4(kShaderUniform::MVP, uniforms.mvp);
+}
+
+inline void setup_legacy_canvas_draw_merge_checkerboard_shader(
+    const LegacyCanvasDrawMergeCheckerboardUniforms& uniforms)
+{
+    setup_legacy_canvas_draw_merge_checkerboard_shader(uniforms, legacy_shader_manager_draw_merge_execution());
+}
+
+inline void setup_legacy_canvas_draw_merge_texture_shader(
+    const LegacyCanvasDrawMergeTextureUniforms& uniforms,
+    const LegacyCanvasDrawMergeShaderExecution& execution) noexcept
+{
+    execution.use_shader(kShader::Texture);
+    execution.set_int(kShaderUniform::Tex, uniforms.texture_slot);
+    execution.set_mat4(kShaderUniform::MVP, uniforms.mvp);
+}
+
+inline void setup_legacy_canvas_draw_merge_texture_shader(
+    const LegacyCanvasDrawMergeTextureUniforms& uniforms)
+{
+    setup_legacy_canvas_draw_merge_texture_shader(uniforms, legacy_shader_manager_draw_merge_execution());
+}
+
+} // namespace pp::panopainter