The testing_only field is not referenced anymore with vaglx removed and
the previous commit dropping all uses.
The ra_hwdec_driver.api field became unused with the previous commit,
but all hwdec interop drivers still initialized it.
Since this touches highly OS-specific code, build regressions are
possible (plus the previous commit might break hw decoding at runtime).
At least hwdec_cuda.c still used the .api field, other than initializing
it.
This commit allows to use the AV_PIX_FMT_DRM_PRIME newly introduced
format in ffmpeg that allows decoders to provide an AVDRMFrameDescriptor
struct.
That struct holds dmabuf fds and information allowing zerocopy rendering
using KMS / DRM Atomic.
This has been tested on RockChip ROCK64 device.