[appkit] Change some NSOpenGL* types to be thread safe. Fixes #60826 (#3476)

Our thread safe checks ensure we're running on the main thread. This is
not the model used by OpenGL where the "same thread" must be used by
the context (but it does not need to be the main one)

https://developer.apple.com/library/content/documentation/GraphicsImaging/Conceptual/OpenGL-MacProgGuide/opengl_threading/opengl_threading.html#//apple_ref/doc/uid/TP40001987-CH409-SW1

https://bugzilla.xamarin.com/show_bug.cgi?id=60826

src/appkit.cs

@@ -8201,6 +8201,7 @@ namespace AppKit {
 		//NSFetchRequest DefaultFetchRequest { get; }
 	}
 
+	[ThreadSafe]
 	[BaseType (typeof (NSObject))]
 	interface NSOpenGLPixelFormat : NSCoding {
 		[Export ("initWithData:")]
@@ -8220,6 +8221,7 @@ namespace AppKit {
 		CGLPixelFormat CGLPixelFormat { get; }
 	}
 
+	[ThreadSafe]
 	[Availability (Deprecated = Platform.Mac_10_7)]
 	[BaseType (typeof (NSObject))]
 	interface NSOpenGLPixelBuffer {
@@ -8249,6 +8251,7 @@ namespace AppKit {
 		int TextureMaxMipMapLevel { get; } /* GLint = int32_t */
 	}
 
+	[ThreadSafe] // single thread - but not restricted to the main thread
 	[BaseType (typeof (NSObject))]
 	[DisableDefaultCtor] // warns with "invalid context" at runtime
 	interface NSOpenGLContext {