Bug 748816. Set the swap interval to 1 on OS X. r=bgirard

This fixes the tearing we get on OS X with OpenGL layers.
It does cause us to occasionally block waiting for vsync.
This isn't ideal but I think it's better than tearing.
OMTC should help us remove this wait.

--HG--
extra : rebase_source : 97739bb0a1a5094ff92e26f5513558ae389ae560

gfx/gl/GLContextProviderCGL.mm

@@ -148,6 +148,8 @@ public:
 
         if (mContext) {
             [mContext makeCurrentContext];
+            GLint swapInt = 1;
+            [mContext setValues:&swapInt forParameter:NSOpenGLCPSwapInterval];
         }
         return true;
     }