Attempt to draw lazily the non-animated tiles. Not working yet.

The goal is to speed up map loading. Most of the time is spent pre-drawing
non-animated tiles.
Fix Grid::get_elements
Non-animated tiles are now drawn lazily, partitioned in a grid.

Cells of this grid are drawn the first time the camera gets close to
them.

Loading a big map now takes 7 ms instead of 300 ms on my machine.
Merge branch 'master' into grid_draw

Conflicts:
	src/entities/NonAnimatedRegions.cpp
Correctly show animated tiles overlapping non-animated ones.