Extract final canvas wrappers and preview mix pass

src/legacy_node_stroke_preview_draw_services.h

@@ -45,6 +45,22 @@ struct LegacyNodeStrokePreviewLiveRenderRequest {
     std::function<void()> draw_composite;
 };
 
+struct LegacyNodeStrokePreviewMixPassExecutionRequest {
+    const Brush& brush;
+    glm::vec2 preview_size {};
+    RTT& mixer_rtt;
+    glm::vec2 bb_min {};
+    glm::vec2 bb_sz {};
+    Sampler& linear_sampler;
+    Texture2D& background_texture;
+    Texture2D& stroke_texture;
+    Texture2D& dual_texture;
+    std::function<void()> draw_mix;
+};
+
+[[nodiscard]] bool execute_legacy_node_stroke_preview_mix_pass(
+    const LegacyNodeStrokePreviewMixPassExecutionRequest& request);
+
 [[nodiscard]] bool execute_legacy_node_stroke_preview_live_render_passes(
     const LegacyNodeStrokePreviewLiveRenderRequest& request);