FFmpeg: Add CUDA hw-texture converter for NVDEC decode on Linux

Adds a texture-converter backend for AV_PIX_FMT_CUDA frames,
copying decoded NV12/NV21/P010/P016 planes directly from CUDA
device memory into a GL- or Vulkan-registered texture. It is not
zery-copy, but GPU-to-GPU copy.

Change-Id: I571bb2fd6dc8344c1f29a8db56fa6b50b3774aa4
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>