collisions betwen entity
blasts erase also enemy projectiles
mobiles now die if too far from screen
max scene interval update (no frame skip if lagging)
tick_busy_loop for precise interval update
now stick exactly on border when trying to fly out of screen
centered projectiles
Merge branch 'upgradeBis' of https://github.com/Agralove/shooter into weapon
too much autoshoot fornow
Conflicts:
	TODO
	entity.py
	parameters.py
sprite preparation
no print
11 levels upgrade for spreader