configure: fix VDA autodetection based on FFmpeg support

The original condition was too weak, requiring only the header. The header is
installed is FFmpeg regardless of the presence of VDA on the system, so just
perform a check on the `ff_vda_create_decoder` function.
cocoa_common: autohide dock when autohiding menubar

A cocoa bug doesn't allow to do otherwise. Will open a radar later.