loaders: Rewrite Exif loader and meta data storage

Now all loader functions have Ex variant that has data storage parameter
to store image metadata to.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
JP2: Fix loader gettting stuck on no end marker.

In case that the file ended up prematurely the read callback will spin
in infinite loop because the read has returned zero. Change it to return
-1 in this case which causes the openjpeg loader abort instead of the
infinite loop.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>