in Gimp and Photoshop- "Too many channels in layer: 4907"

To be better able to handle 64 bit offsets we switch to
using GInputStream/GOutputStream and its related
functions instead of using FILE functions like fseek.

In addition we changed several offsets and sizes to
64 bit instead of 32 bit to correctly handle large
image sizes and offsets.

I should probably have split this out in two parts
(the move to GInput/OutputStream, and the
changing to 64 bit variables) but don't feel like
it's worth the trouble at this point.