Add NamedMultiTextureAtlas and IndexedMultiTextureAtlas.

[Possseidon]

Since MultiTextureAtlas is templated, it cannot be created dynamically with different sizes, which might be necessary in scripting or some other rare use cases in which the size is not known at compile time.

Therefore NamedMultiTextureAtlas shall use string keys for the different SubTextures (using a map) and IndexedMultiTextureAtlas shall use integer keys in the form of std::size_t (using a vector).

The names/count should be given at construction however, as once the first texture is added, it does not make much sense to add another SubTexture afterwards, so we might as well give it directly at construction.