Switched to using AVFoundation and VideoToolbox.

There's still a bit of a mess left over, and you can't change any
encoder settings yet, but it's working for what it is.
Require 10.8 or later, not 10.6 or later.

VideoToolbox is available only in 10.8 or later, at the least.
Lots more options for controlling compression settings.

Mostly untested at this point.
Keep default compression settings separate, so that we don't ever complain if they can't be applied.

As distinct from the user-specified settings, of which any failures to
enact them should be noted to the user.