Extract preview mix pass execution

src/node_stroke_preview.h

@@ -73,6 +73,11 @@ public:
         bool copy_stroke_destination,
         float zoom,
         const glm::vec2& size);
+    pp::panopainter::LegacyNodeStrokePreviewMixExecutionRequest make_stroke_draw_mix_execution_request(
+        const pp::panopainter::LegacyNodeStrokePreviewMixPassPlan& mix_pass,
+        gl_state& gl,
+        const glm::vec2& bb_min,
+        const glm::vec2& bb_sz);
     void draw_stroke();
     void draw_stroke_immediate();
     Image render_to_image();