The 'simple' hwaccels (not h.264 and hevc) all use the same bitstream
management and reference lookup logic so let's refactor all that into
common functions.
I verified that casting a signed int -1 to unsigned char produces 255
according to the C language specification.
Some parts of the code are based on a patch by
Timo Rothenpieler <timo@rothenpieler.org>
Merges Libav commit b9129ec466.
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>
