Document env vars for video decoding #65
Description
Some video encoders / decoders are only enabled through env vars.
Global
The Vulkan Video Decoding specs only cover H.264, H.265, and AV1. Older codecs will require you to use your traditional hardware decoding API (VAAPI, nvdec, dx11va, etc) for now, and possibly forever - depending on whether effort is put into exposing older codecs.
Intel
You must set ANV_VIDEO_DECODE=1 in your environment to expose video decoding
Both H.264 and H.265 are supported
VK_EXT_descriptor_buffer support, required for Vulkan filters, has been [merged](https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22151) but no release includes it yet
AMD
You must set RADV_PERFTEST=video_decode in your environment to expose video decoding
Both H.264 and H.265 are supported
Vulkan filters are supported
Nvidia
Both H.264 and H.265 are supported
AV1 is supported with > 550.54.14 drivers
Vulkan filters are supported