cocoa: fix native fullscreen

This fixes a couple of issues with the Cocoa `--native-fs` mode, primarily:

 - A ghost titlebar at the top of the screen in full screen:

   This was caused by the window constraining code kicking in during
   fullscreen. Simply returning the unconstrained rect from the constraining
   method fixes the problem.

 - Incorrect behavior when using the titlebar buttons to enter/exit
   fullscreen, as opposed to the OSD button.

   This was caused by mpv's internal fullscreen state going out of sync with
   the NSWindow's one. This was the case because `toggleFullScreen:`
   completely bypassed the normal event flow that mpv expects.

Signed-off-by: Ryan Goulden <percontation@gmail.com>

Change style for mpv, simplify and refactor some of the constraining code.

Signed-off-by: Stefano Pigozzi <stefano.pigozzi@gmail.com>