Set default document res to 6K. Implement blending on canvas, mixer and export equirect.

src/node_canvas.h

@@ -6,6 +6,9 @@ class NodeCanvas : public Node
 {
 public:
     std::unique_ptr<Canvas> m_canvas;
+    RTT m_blender_rtt;
+    RTT m_cache_rtt;
+    Texture2D m_blender_bg;
     Sampler m_sampler;
     Sampler m_sampler_linear;
     Sampler m_sampler_stencil;