draw main brush preview first if the queue is long

src/node_panel_stroke.cpp

@@ -428,6 +428,8 @@ void NodePanelStroke::init_controls()
 
 
     m_preview = find<NodeStrokePreview>("canvas");
+    m_preview->m_draw_first = true;
+
     m_blend_mode = find<NodeComboBox>("blend-mode");
     m_blend_mode->on_select = [this](Node*, int index) {
         Canvas::I->m_current_brush->m_blend_mode = index;