Add separate options to control whether we build VA-API and/or V4L2
hardware video decode.  These options default to enabled on the
platforms where these decoders are used.

The downstream MOZ_WAYLAND_USE_HWDECODE build option is used within the
FFmpeg platform and is only set when building against a supported
version of the FFmpeg libraries (i.e. not for ffmpeg57).

Differential Revision: https://phabricator.services.mozilla.com/D178945