vr brush preview and pen gestures (pick and size)

src/canvas_modes.h

@@ -38,6 +38,7 @@ public:
 
 class CanvasModePen : public CanvasMode
 {
+    friend class App;
     bool m_draw_tip = false;
     bool m_dragging = false;
     glm::vec2 m_pan_start;