h264dec: add a NVDEC hwaccel

Some parts of the code are based on a patch by
Timo Rothenpieler <timo@rothenpieler.org>

Merges Libav commit b9129ec4668c511e0a79e25c6f25d748cee172c9.

Due to the name clash with our cuvid decoder, rename it to nvdec.

This commit also changes the Libav code to dynamic loading of the
cuda/cuvid libraries.

Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>
