H264 DXVA2 implementation

 It allows VLD H264 decoding using DXVA2 (GPU assisted decoding API under
VISTA and Windows 7).
 It is implemented by using AVHWAccel API. It has been tested successfully
for some time in VLC using an nvidia card on Windows 7.

 To compile it, you need to have the system header dxva2api.h (either from
microsoft or using http://downloads.videolan.org/pub/videolan/testing/contrib/dxva2api.h)
 The generated libavcodec.dll does not depend directly on any new lib as
the necessary objects are given by the application using FFmpeg.

Originally committed as revision 21353 to svn://svn.ffmpeg.org/ffmpeg/trunk