Share retained stroke preview mix executor

src/legacy_node_stroke_preview_execution_services.h

@@ -87,6 +87,24 @@ struct LegacyNodeStrokePreviewMixPassRequest {
     int blend_mode = 0;
 };
 
+struct LegacyNodeStrokePreviewMixExecutionRequest {
+    LegacyNodeStrokePreviewMixPassPlan::ShaderPlan shader {};
+    int mixer_width = 0;
+    int mixer_height = 0;
+    int scissor_x = 0;
+    int scissor_y = 0;
+    int scissor_width = 0;
+    int scissor_height = 0;
+    std::function<void()> save_state;
+    std::function<void(const LegacyNodeStrokePreviewMixPassPlan::ShaderPlan&)> setup_mix_shader;
+    std::function<void()> bind_mixer_framebuffer;
+    std::function<void(int, int, int, int, int, int)> configure_mix_target_state;
+    std::function<void()> bind_mix_inputs;
+    std::function<void()> draw_mix;
+    std::function<void()> unbind_mixer_framebuffer;
+    std::function<void()> restore_state;
+};
+
 [[nodiscard]] inline LegacyNodeStrokePreviewMixPassPlan plan_legacy_node_stroke_preview_mix_pass(
     const LegacyNodeStrokePreviewMixPassRequest& request) noexcept
 {
@@ -130,6 +148,39 @@ struct LegacyNodeStrokePreviewMixPassRequest {
     return plan;
 }
 
+[[nodiscard]] inline bool execute_legacy_node_stroke_preview_mix_pass(
+    const LegacyNodeStrokePreviewMixExecutionRequest& request)
+{
+    if (request.mixer_width <= 0 ||
+        request.mixer_height <= 0 ||
+        !request.save_state ||
+        !request.setup_mix_shader ||
+        !request.bind_mixer_framebuffer ||
+        !request.configure_mix_target_state ||
+        !request.bind_mix_inputs ||
+        !request.draw_mix ||
+        !request.unbind_mixer_framebuffer ||
+        !request.restore_state) {
+        return false;
+    }
+
+    request.save_state();
+    request.setup_mix_shader(request.shader);
+    request.bind_mixer_framebuffer();
+    request.configure_mix_target_state(
+        request.mixer_width,
+        request.mixer_height,
+        request.scissor_x,
+        request.scissor_y,
+        request.scissor_width,
+        request.scissor_height);
+    request.bind_mix_inputs();
+    request.draw_mix();
+    request.unbind_mixer_framebuffer();
+    request.restore_state();
+    return true;
+}
+
 struct LegacyNodeStrokePreviewPassSequenceRequest {
     bool dual_pass_enabled = false;
     std::function<void()> prepare_dual_pass;