Not that C++ STL and exceptions would be anything to take inspiration
from, but there's std::out_of_range. Python IndexError is also specified
as "index out of range", not "bounds".
About time this got done. This also has an XFAIL for the case where a
distance field image is processed with an offset, have to fix the
underlying issue in TextureTools first.
Also added a range assertion for the distance field image setter to
match what the abstract base does, together with a corresponding getter.
Allows the Font and FontConverter plugins be built without TARGET_GL
enabled. That was the last piece missing for making the magnum-plugins
repo completely GL-free.