It's no longer a good idea to use a single callback instance since we switched to using ShaderConstantID's.
Those id's are really indices to arrays - and each added material has it's own array. We basically
used indices to arrays of another object (which happened to be identical in this case).
And this starts to go horribly wrong once the code of one shader is slightly modified...
git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@5485 dfc29bdd-3216-0410-991c-e03cc46cb475