refactor to have a single global current brush on Canvas::m_current_brush and make panels read from it for drawing instead of relying on their own copy

engine/canvas.h

@@ -88,6 +88,8 @@ public:
     glm::vec3 m_cam_pos;
     float m_cam_fov = 85;
 
+    Brush m_current_brush;
+
     enum class kCanvasMode { Draw, Erase, Line, Camera, Grid, Fill, MaskFree, MaskLine, COUNT };
     kCanvasMode m_state{ kCanvasMode::Draw };
     static std::vector<CanvasMode*> modes[];