travis: use homebrew to build ffmpeg on OS X

This allows us to use a newer version of ffmpeg and to test the build of our
VDA code.
vda: only support the new hwaccel 1.2 API (remove old code)

Since the new hwaccel API is now merged in ffmpeg's stable release, we can
finally remove support for the old API.

I pretty much kept lu_zero's new code unchanged and just added some error
printing (that we had with the old glue code) to make the life of our users
less miserable.