For integer attributes, that is. This never worked and the MeshGLTest
even had an explicit test case for this, but it took me until now to
realize that this would be best caught at compile time. So now it is.
But since this functionality dates back to 2010, just removing the
values would likely break a lot of code (well, broken code, but still),
so the disallowed values are now deprecated, giving a compile time
warning when used, and are removed only on non-deprecated builds.
Vladimír Vondruš