vout_macosx: remove misleading user-facing error message about Quartz Extreme being unavailable (close #9777)

While it is strongly desirable to be present, the API behavior or the driver quality seem to have changed on OS X 10.9, so this alert is triggered way to often while video is actually being displayed just fine.

Log message is retained for debugging purposes.
avformat mux: use a variable
macosx/about: fix layout issue and simply code (close #10746)
Merge 2.1.2 and 2.1.3 NEWS