Improvements in loading broken xcf files, based on examining issue #8230.
Besides checking for a minimum width and height, GIMP also has a maximum
size we can and should check.

In the case of the image itself, we change invalid dimensions to a size of
1 in hope that the individual layers etc will have the correct size.
For layer, we will also try to go on, but for channel and layer mask, we
will give up.

(cherry picked from commit 24c962b9)