implement multithreaded rendering with context switch, gl state save/restore, add progress bar ui node, implement stencil texture for brush, implement multithreaded canvas load/save/export pano. Missing multithread in windows.

PanoPainter-OSX/main.h

@@ -0,0 +1,17 @@
+#include <CoreFoundation/CoreFoundation.h>
+#include <Cocoa/Cocoa.h>
+#include <CoreVideo/CoreVideo.h>
+#include <OpenGL/OpenGL.h>
+
+@interface View : NSOpenGLView
+{
+    CVDisplayLinkRef dl;
+    NSOpenGLContext* glctx;
+    _CGLContextObject* cgl;
+    bool gl_ready;
+}
+- (void)terminateGL;
+- (void)async_lock;
+- (void)async_unlock;
+- (void)async_swap;
+@end