Add README files for the vaapi, intel, nvidia, and vulkan encoder packages
covering the FFmpeg flags, devices, supported formats, required system packages,
and FFmpeg 8 notes such as the VA-API filter-device requirement.
encode.FormatNV12 already ends in hwupload, so appending it again produced
"format=nv12,hwupload,hwupload", which fails because the frames are already on
the GPU. Build the filter from FormatNV12 directly to upload exactly once, and
add a gated Vulkan case to the transcode command tests.