This relies on !20384 (merged) landing first.
This contains an implementation of the vulkan video decode extensions for H264/H265. It has a runtime guard of (RADV_VIDEO_DECODE=1 envvar) at the moment.
This isn't conformant, because I think I'm hitting some firmware limitations on interleaving, but it does pass the normal decode tests and works with ffmpeg implementation.
This also contains some common vulkan video runtime code, not sure if it's worth dealing with that in a separate MR or not.