On multi-GPU boxes VAAPI's default render node can point at a device
with no VA driver (NVIDIA without libva-nvidia-driver), failing before
the working AMD/Intel node is ever tried; trying CUDA (NVDEC) first is
both the discrete-GPU path and sidesteps that misdirection. VAAPI stays
as the fallback for AMD/Intel-only machines.
Adds a hwcheck example printing which hw device a stream opens with and
the raw av_hwdevice_ctx_create result codes per device type.
