This is my first patch submission, so please let me know if I'm doing
something wrong.

This patch fixes problems with redraws and resizes when a gtkimage is
within a container that might give it an allocation larger than the
actual image.  It fixes both screen glitches and actual crashes (the
crashes generally only occurred when using xshm).

I believe that the basic problem was that the old code really needed
to be doing a full rectangle intersection instead of more limited
checks.  This patch does not fix a cosmetic but ugly "resize not
clearing the right areas on redraw" problem, but I'll look at that
later.

I don't know if it's a violation of the Gtk coding standard, but I
moved some variables inside the "if's" local scope so they wouldn't
take up stack space when not needed (assuming that the optimizer
doesn't kill them anyway in that situation), and so that you can't
accidentally refer to them when you didn't mean to.  If that's not OK,
please let me know so I don't do it in the future.

Thanks

Rob Browning <rlb@cs.utexas.edu>
PGP fingerprint = E8 0E 0D 04 F5 21 A0 94  53 2B 97 F5 D6 4E 39 30