enhanced entity rendering

[FreddyMSchubert]

One ray can only render one entity. Thus, you can't see other entities through the transparent bits of walls, and many entities look terrible because they hide each other.

Instead, a given ray should be able to render all entities in it's way in order of distance.

[FreddyMSchubert]

Entity rendering was properly reworked. It takes more performance now but is fully functional as you'd expect, and as entities are kind of all you interact with in the game, I reckon it's justified.
Merging with master!