cleaner/faster grid physics.

[andrew-peng-144]

WOW! dont always iterate thru each tile every time. for a 50x50 its already ~2500 wasted computations per frame.

[andrew-peng-144]

currently using 1d array of "buckets", each bucket having 10 numbers max representing the entities in that cell. possible solutions:

• Keep a set of visited cells for this frame, and only check the visited cells for collision. Worst case is possibly worse than current, but in practice no more than 100 out of the 2500 cells would be filled anyway so this saves computation most of the time
• Use quadtree instead or some other data struct instead

[andrew-peng-144]

or just use fewer cells. Cell size increased to 32x32 instead of 16x16 (unzoomed) pixels, this lowers number of cells by factor of 4, but has a lower maximum # of entities.