Fixes blending differences in test_brush/test_sprite maps. They now look
fairly similar (modulo color correction).
OIT is still a bit off, but good enough for now
They have mismatching frequencies. ModelMetadata should be per-Model, there should be only a few dozen of these.
There maybe hundreds (or even thousands) of materials, but one material can be still referenced by many kusochki.
This only moves fields into new structs, which still belong to Kusok. The real extraction will happen later, see #52.