This is modelled similarly to the password registry API.
We have an array to which new handlers can be added, and when a built-in
handler cannot handle the image, we try the handlers in the array.
The standard module is in control of registering a new constant for the
image file type so that no clashes can occur. It also updates the image
file type count constant. As such, the registration may only happen
during module startup.

This implements an SVG handler using the libxml reader API. This does
not parse the entire document but instead uses a pull parser to locate
the root element, check whether it's an svg root, do some extra sanity
checks on the attribute, and fill in the php_gfxinfo structure.

… API