Mac: Immediately update current OpenGL context's shape.

Previously we were postponing our -[NSOpenGLContext update] call to the next
SDL_GL_SwapWindow, even if the context was current on the current thread. This
changes it so that we will do the update immediately if it's the current
context.

If you're rendering on another thread, you need to call SDL_GL_SwapWindow once
after a resize event to ensure your drawable will produce non-garbage data.

Bug: https://bugzilla.libsdl.org/show_bug.cgi?id=2339

src/video/cocoa/SDL_cocoaopengl.m

@@ -110,13 +110,21 @@
 
         if ([self view] != [windowdata->nswindow contentView]) {
             [self setView:[windowdata->nswindow contentView]];
+            if (self == [NSOpenGLContext currentContext]) {
+                [self update];
+            } else {
                 [self scheduleUpdate];
             }
+        }
     } else {
         [self clearDrawable];
+        if (self == [NSOpenGLContext currentContext]) {
+            [self update];
+        } else {
             [self scheduleUpdate];
         }
     }
+}
 
 @end
 

src/video/cocoa/SDL_cocoawindow.m

@@ -49,13 +49,18 @@ static void ConvertNSRect(NSRect *r)
 static void
 ScheduleContextUpdates(SDL_WindowData *data)
 {
+    NSOpenGLContext *currentContext = [NSOpenGLContext currentContext];
     NSMutableArray *contexts = data->nscontexts;
     @synchronized (contexts) {
         for (SDLOpenGLContext *context in contexts) {
+            if (context == currentContext) {
+                [context update];
+            } else {
                 [context scheduleUpdate];
             }
         }
     }
+}
 
 static int
 GetHintCtrlClickEmulateRightClick()