implement unit breathing

possible side effect of this change:
becaues we are now calling RNG multiple times per frame at depths greater
than 0, any kind of RNG-related reproducability goes rightout the window.
this is a potentially bad thing.

also this will probably need adjustment for varying unit heights.
dead guys don't breathe

granted they're drawn as items instead and as such don't have bubbles drawn
but they may as well not call RNG or anything while they're down as well.
tidy breathing code a little.
speed up base defenses, apply damage ranges, and play more splosions
missed a spot
speed up unit breathing/no bubbles undercover

the bool to switch frame updates was in the code, but didn't alter the speed
of the bubble frame updates, they move at the same speed as regular animations

bubbles should also not be drawn if the unit is under a visible roof
add comments.