accel: refactor to fit new hwaccel api

libavutil now covers the simple cases for creating and managing devices.
As Ring does not need to fine-tune these processes, most of the accel
code can be and has been removed.

Most hardware decoders output NV12, so skip extra conversions by
outputting NV12. Said pixel format is supported by everything that isn't
excessively old.

Change-Id: I10c440026fc3b289dbba7ecbca47e55c57147207
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>