Alternate devIL source

[ptheywood]

Source forge downloads occasionally cause CI jobs to fail, which is annoying.

#102 / #103 resolve this for GLEW, but devIL is still a sourceforge based download, so may continue to cause sporadic CI issues. If this is the case, we could do with an alterantive hosted location for devIL.

I've been unable to find a mirror of the pre-compiled binaries for devIL, but we could potentially host our own mirror via github if required (either via CI to create a release, or to just manually create a release and attach the precompiled zip?). It is LGPL so this should be fine to do, we'd just need to make it clear that it's a mirror in the repo / release description.

There might be a better / less faffy alternative available already that i'd just not found on a quick look.