copy: minor speed enhancement to USWC copy

Adopt suggestions as per https://software.intel.com/en-us/articles/copying-accelerated-video-decode-frame-buffers:
- 64kB aligned memory buffer
- Call to mfence only around copy to cache routine.

Use a single instruction to copy non-aligned memory at startup.
Remove compilation warning on mac

This result in a small 1.3% speed increase on an i7-4650U

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
vaapi: prefer vaGetImage over vaDeriveImage under most circumstances.

While vaDeriveImage is slightly faster than vaGetImage, there's an added cost of having to convert NV12 into YV12 later, which in practice negates any benefits.
Prefer YV12 and YUV420 formats over NV12.
This decrease time to process a VAAPI frame by over 50%: from 2.1ms/frame to 0.9ms/frame on an i7-4650U

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>