avformat/img2: Make AVOptions available to img* demuxers defined in other files

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/img2_brender_pix: Add AVClass and image2 AVOptions

This fixes the default timebase and allows overriding the framerate

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/img2_alias & brender_pix: fix function names

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>