ffmpeg_opt: rename loop option to stream_loop

The "loop" option is used in several demuxers (like img2dec) and muxers, using the same name in ffmpeg_opt
breaks them. Feel free to revert this and replace by any other solution or rename both as preferred
This is just as a quick fix to avoid the regression with existing command lines and to have both named
the same (which does not work)

Example:
./ffmpeg -loop 1 -i fate-suite/png1/lena-rgb24.png -t 1 test.avi
will produce 25 frames with the img2dec loop but only 1 frame at 25fps with the ffmpeg loop option

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>