[Cin] selectable vaapi decoder device

Because I have this dual-gpu laptop with intel and radeon (r600) decoding devices I tried to get second (mesa/r600) device going but init failed .. I looked into decoding example: https://gist.github.com/kajott/d1b29c613be30893c855621edd1f212e and created attached patch now you can select your specific device with env variable: CIN_DRM_DEC=/dev/dri/renderD129 DRI_PRIME=1 LIBVA_DRIVER_NAME=r600 bin/cin in theory same treatment can be applied (with different variable name) to second instance of av_hwdevice_ctx_create (in hw encoder init) but I have no device to test it ...