video_filter: add formatcrop filter

This filter crops the input pictures by adjusting the format (offsets and size), without any copy (contrary to croppadd).

This is especially useful to receive pictures with arbitrary visible area (smaller than the full size), to debug vouts or filters.

Example usage:

./vlc --video-filter='formatcrop{left=50,right=100,top=200,bottom=80}' video.mp4