Add a hwdevice type for V4L2 Request API with transfer_data_from support
for AV_PIX_FMT_DRM_PRIME, based on AV_HWDEVICE_TYPE_DRM.
AVV4L2RequestDeviceContext.media_fd can be set by the application or a
media device path can be supplied when hwdevice is created. When none
is supplied it default to -1 and hwaccel will auto-detect a media device
with a capable video device.
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
