lua: subprocess: fix dumb
dxva2: use optimized memcpy

At least on my machine, reading back the frame with system memcpy is
slower than just using software rendering. Use the optimized gpu_memcpy
from LAV to speed things up.
video: clarify what IMFMT_DXVA2 is
vo_direct3d: support nv12/nv21 directly
dxva2: gpu_memcpy: fix build for GCC 4.8.3
manpage: options: various fixes
TOOLS/lua/autoload: don't shadow local variable

"dir" is already used somewhere above. This was ok, but not nice.
TOOLS/lua/autoload: fix operation outside of working dir

Fixes #1222. (This commit is based on a patch posted there.)
dxva2: fix crash on initialization failure

If dxva2_init() fails, dxva2_uninit() will be called twice.