avformat/img2dec: make image2dec capable to be used from seperate demuxers

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat: add image2 alias pix demuxer

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat: add image2 brender pix demuxer

Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
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>