cocoa: OpenGL setView and update must be used on main thread (thanks, Tim!).

If called from background threads, use Grand Central Dispatch to use the
main thread instead. On the main thread, just call them directly.

Fixes Bugzilla #4932.

src/video/cocoa/SDL_cocoaopengl.h

@@ -49,6 +49,8 @@ struct SDL_GLDriverData
 - (void)scheduleUpdate;
 - (void)updateIfNeeded;
 - (void)setWindow:(SDL_Window *)window;
+- (SDL_Window*)window;
+- (void)explicitUpdate;
 
 @end
 

src/video/cocoa/SDL_cocoaopengl.m

@@ -63,14 +63,10 @@
 /* This should only be called on the thread on which a user is using the context. */
 - (void)updateIfNeeded
 {
-    int value = SDL_AtomicSet(&self->dirty, 0);
+    const int value = SDL_AtomicSet(&self->dirty, 0);
     if (value > 0) {
         /* We call the real underlying update here, since -[SDLOpenGLContext update] just calls us. */
-        if ([NSThread isMainThread]) {
-            [super update];
-        } else {
-            [super performSelectorOnMainThread:@selector(update) withObject:nil waitUntilDone:NO];
-        }
+        [self explicitUpdate];
     }
 }
 
@@ -119,9 +115,13 @@
         }
 
         if ([self view] != contentview) {
+            if ([NSThread isMainThread]) {
                 [self setView:contentview];
+            } else {
+                dispatch_sync(dispatch_get_main_queue(), ^{ [self setView:contentview]; });
+            }
             if (self == [NSOpenGLContext currentContext]) {
-                [self update];
+                [self explicitUpdate];
             } else {
                 [self scheduleUpdate];
             }
@@ -129,13 +129,27 @@
     } else {
         [self clearDrawable];
         if (self == [NSOpenGLContext currentContext]) {
-            [self update];
+            [self explicitUpdate];
         } else {
             [self scheduleUpdate];
         }
     }
 }
 
+- (SDL_Window*)window
+{
+    return self->window;
+}
+
+- (void)explicitUpdate
+{
+    if ([NSThread isMainThread]) {
+        [super update];
+    } else {
+        dispatch_sync(dispatch_get_main_queue(), ^{ [super update]; });
+    }
+}
+
 @end
 
 
@@ -352,8 +366,10 @@ Cocoa_GL_MakeCurrent(_THIS, SDL_Window * window, SDL_GLContext context)
 {
     if (context) {
         SDLOpenGLContext *nscontext = (SDLOpenGLContext *)context;
+        if ([nscontext window] != window) {
             [nscontext setWindow:window];
             [nscontext updateIfNeeded];
+        }
         [nscontext makeCurrentContext];
     } else {
         [NSOpenGLContext clearCurrentContext];