avformat/img2dec: Use avformat probing interface to identify format if it has not been otherwise identified

This is used only for distinguishing .pix formats for now.
Which is the only case that has image2 demuxers currently
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>