Cocoa: Hide cursor instead of using blank image

When cursor isn't in normal mode and should be hidden, use [NSCursor hide]
method instead of setting it to blank image. This should prevent
situations when hidden cursor becomes visible after system notification
was shown.

Fixes #971.
Closes #1028.

src/cocoa_init.m

@@ -352,9 +352,6 @@ void _glfwPlatformTerminate(void)
         _glfw.ns.listener = nil;
     }
 
-    [_glfw.ns.cursor release];
-    _glfw.ns.cursor = nil;
-
     free(_glfw.ns.clipboardString);
 
     _glfwTerminateNSGL();