Use Zend allocator methods in gd

[YakoYakoYokuYoku]

Previously when using system gd there was no way to use the Zend allocator methods to limit its memory usage.
This PR fixes this by using the new memory allocator setters in libgd/libgd#692. Also, this PR gives the possibility to unbundle ext/gd/libgd.

[iluuu1994]

Thank you @YakoYakoYokuYoku!

[devnexen]

I definitively like your suggestion, let's see what @kocsismate think of it.

[cmb69]

Thank you for the PR, @YakoYakoYokuYoku! This is certainly nice to have, but I guess we need to wait for libgd/libgd#692 to be merged (and probably until it gets released).

Also, this PR gives the possibility to unbundle ext/gd/libgd.

While I, personally, would like to do this as soon as possible, we can only do this for a new major or minor PHP release, and I'm afraid it's way too late for PHP 8.4. At least for the Windows builds, we would need a libgd version which supports the custom allocators, and users of other platforms who build against the bundled libgd would need to be informed of that in advance. And of course, unbundling libgd would need an RFC.

[YakoYakoYokuYoku]

No worries as I'm not in a hurry, in fact said PR was long accepted. Happens that it hasn't been merged because we detected an error during testing. I've fixed it since then but the upstream maintainers are ignoring it, I don't know why, perhaps it's due to work I guess. To be fair it went off my radar, though I hope it gets merged anytime soon.

At least for the Windows builds, we would need a libgd version which supports the custom allocators, and users of other platforms who build against the bundled libgd would need to be informed of that in advance. And of course, unbundling libgd would need an RFC.

Also this PR doesn't force packagers to use the most recent version of libgd because there are conditionals that get only turned on when you use a version that supports the custom allocators. Due to this, every other version still remains supported too.