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.
We add PHP bindings for libgd's features to read and write BMP files, which
are available as of libgd 2.1.0.
As PHP's bundled libgd doesn't yet include the respective features of the
external libgd, we add these.
