Implementation for texture level parameter query.
Fixed name clash in internal implementation.

Clang didn't like that.
Don't instanitate/declare structs as classes.

Clang warned about that.
Template aliases also can't have "conflicting" default arguments.

Clang didn't like that (and I thought these workarounds aren't needed
with C++11's alias template anymore).
Clang fixes for friend declarations.

Friending only constructor apparently doesn't help, specifying
full-scope name for Resource.
Clang fixes for templated base classes.

Unlike GCC it does need the `template` keyword on weird places.
Clang also needs parentheses around default templated arguments.
Clang linker fixes.
Ability to retrieve texture image size in given mip level.
Text: proper UTF-8 support in Font::prerender().
TextureTools: use Math::max() instead of doing it by hand.