Crafted BMP images can cause dynamicSeek() to be called with a negative
position which must not be allowed, since dynamicSeek() works like
fseek() in SEEK_SET mode. We solve this by bailing out if `pos` is
negative, and let the image reading fail gracefully.
Fixed memory in leak ImageCreateFromString().
Fixed a bug that prevented proper identification of WBMP images and thus
prevented ImageCreateFromString() from working on those images.
This initial checkin has no changes to any of the libgd code so it can
be used as a basis for diffs. It also will not build currently because
of this. The PHP gd checks need to be incorporated along with a bit of
other config magic. It also shouldn't break the build and will only
take effect if you use --with-gd=php right now.
