Trying to use the display server as the QSV device was found to be
generally wrong in beta, so instead lets save defaults from the earlier
device enumeration similar to obs-ffmpeg-vaapi which is known to work
well.
VPL does not have a consistent initialization process for the various
backends it dispatches to. So this resolves the new VPL dispatcher
failing to initialize the video session when it dispatches to
intel-mediaSDK backend. On the oneVPL-intel-gpu backend VA-API sessions
are correctly initialized so this wasnt noticed when tested on newer
hardware.
This moves the existing adapter checking into the platform layer and
moves the Windows implementation to its platform implementation and adds
a Linux implementation based on directly querying VA-API.
Unlike Windows, this check takes ~1ms so we have no need to spin out
another thread to perform the work. This also fixes up some of the CPP/C
mixing going on in common_utils."h" to allow us to call common functions
from C files.
