hwcontext_d3d11va: add option to enable debug mode

Basically copied from VLC (LGPL):

http://git.videolan.org/?p=vlc.git;a=blob;f=modules/video_output/win32/direct3d11.c;h=e9fcb83dcabfe778f26e63d19f218caf06a7c3ae;hb=HEAD#l1482
http://git.videolan.org/?p=vlc.git;a=blob;f=modules/codec/avcodec/d3d11va.c;h=85e7d25caebc059a9770da2ef4bb8fe90816d76d;hb=HEAD#l599



Signed-off-by: Luca Barbato <lu_zero@gentoo.org>