draw main brush preview first if the queue is long

src/node_stroke_preview.h

@@ -32,6 +32,7 @@ public:
     static void terminate_renderer();
     std::shared_ptr<Brush> m_brush;
     std::shared_ptr<Brush> m_dual_brush;
+    bool m_draw_first = false;
     Stroke m_stroke;
     Stroke m_dual_stroke;
     std::vector<glm::vec2> m_bez_points;