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/node_canvas.h

@@ -7,7 +7,6 @@ class NodeCanvas : public Node
 public:
     std::string data_path;
     std::unique_ptr<ui::Canvas> m_canvas;
-    ui::Brush m_brush;
     Sampler m_sampler;
     Sampler m_sampler_linear;
     Sampler m_sampler_stencil;