FFmpeg Lands OpenCL Improvements

Besides a lot of NVDEC code landing for the next FFmpeg release, there's also been a number of OpenCL improvements that were just committed to this multimedia library's codebase.

The work landed yesterday includes removing an older and experimental OpenCL API while introducing a new OpenCL "hwcontext" implementation.

This in turn has introduced an OpenCL overlay filter and OpenCL unsharp mask filter.

Along with these new OpenCL filters and new hwcontext API there is support for VA-API to OpenCL mapping with the Intel Mesa driver and Beignet, QSV to OpenCL for the Intel Media SDK, DXVA2 to OpenCL mapping, D3D11 to OpenCL, and DRM to OpenCL. These mappings are about allowing for sharing/interoperability between the OpenCL compute API and the different display/graphics APIs for sharing of data/buffers.

These OpenCL improvements and possibly more CL work will be found in the next FFmpeg release, expected to be FFmpeg 3.5.