avformat/img2dec: reduce bmppipe probe score

bmp pipe needs the bmp parser which is not bug free and should thus not be favored
over the bmp image2 demuxer
that also means this change could be reverted in case bmp pipe is improved so it
handles all single bmp images correctly

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/format: Run image2 probe again when file content data is available

Reviewed-by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>