Cache blending

Blending data is now cached, [issue #154](https://github.com/SFTtech/openage/issues/154)
It's possible (see [possible](https://github.com/schets/openage/commit/a7ca140a650d667cea29acffb8f15e82789a0701)) to do it without the distinction, but the create_tile_advice is fairly cheap without the blending turned on and I wasn't sure if it was really worth the extra memory usage. It may be possible to do this w/o caching data separately for both blended and unblended, in which case I see no reason to avoid the cache in those places.