Cocoa: ignore mouseDown events in a window's titlebar.

These events accidentally slipping in sometimes appears to be a bug (or
maybe new behavior) in 10.10, as previous versions of Mac OS X don't appear
to ever trigger this.

Thanks to Paulo Marques for pointing out the fix on the SDL mailing list!

Fixes Bugzilla #2842 (again).

src/video/cocoa/SDL_cocoawindow.m

@@ -751,6 +751,14 @@ SetWindowStyle(SDL_Window * window, unsigned int style)
 {
     int button;
 
+    /* Ignore events that aren't inside the client area (i.e. title bar.) */
+    if ([theEvent window]) {
+        const NSRect windowRect = [[[theEvent window] contentView] frame];
+        if (!NSPointInRect([theEvent locationInWindow], windowRect)) {
+            return;
+        }
+    }
+
     if ([self processHitTest:theEvent]) {
         return;  /* dragging, drop event. */
     }