gl: Cocoa window is never closed

More technical description

GstGLWindowCocoa fails to close the window it creates.

Reproduction steps

Create a bunch of pipelines outputting to glimagesink with GST_GL_HAVE_WINDOW_COCOA defined. Windows will be created, and they will never be closed.

They will not be closed if the pipeline state is set to NULL nor if the pipeline and glimagesink are deleted.

Expected result

With GST_GL_HAVE_WINDOW_X11 defined the glimagesink element will close the window when the state is set to NULL.

Suspected issue

gstglwindow_cocoa.m#L207 creates the Cocoa window:

  internal_win_id = [[GstGLNSWindow alloc] initWithContentRect:rect styleMask:
      (NSWindowStyleMaskTitled | NSWindowStyleMaskClosable |
      NSWindowStyleMaskResizable | NSWindowStyleMaskMiniaturizable)
      backing: NSBackingStoreBuffered defer: NO screen: nil gstWin: window_cocoa];

I believe that gstglwindow_cocoa.m#L244 should close the window, but instead it just deleted the view from within the window:

[[internal_win_id contentView] removeFromSuperview];

I suspect that the above line should be replaced with:

[internal_win_id close];