Speed up DXVA2

The new DXVA2 hwaccel works for me. Only, with the CRT memcpy, it's slower than software rendering. LAV Video uses an optimised gpu_memcpy function, and this speeds things up a lot on my hardware.

Should I add a configure check for the SSE4 intrinsics in gpu_memcpy_sse4.h? Hopefully no one uses MinGW compilers without them.