Add support for skipping a header for rawvideo inputs

[erezst]

Some programs dump uncompressed video files with a custom header at the beginning. To use these files as inputs for FFMPEG, the rawvideo codec needs to be able to skip this header.

The input format is:

[ Header - e.g. 100 bytes]
[ Frame #1 ]
[ Frame #2 ]
[ ... ]
[ Frame #N ]

Header -- a fixed-length header (for example, 100 bytes).
Frame - A raw video frame in some pix_fmt (e.g. YUV422p).

A command-line for encoding this file to H.264 might be:

ffmpeg -vcodec rawvideo -f rawvideo -pix_fmt yuv422p -skipheader 100 -i <SOURCE_VIDEO> -s 640x480 -r 30 -vcodec libx264 <OUTPUT_FILE>

The '-skipheader 100' parameter is what this feature request is about.

[Carl Eugen Hoyos]

New option -skip_initial_bytes was committed to git head.