messed with gravity a little bit. this is the main thing i want to tinker with before leaving most of the movement stuff the same as before. an arbitrary gravity direction helps generalize a lot of physics calculations and eventually the movement directions. it's not vital but it seems simple enough to include since i already have to handle gravity myself.

this also encourages me to have physics states like grounded, falling, and slipping checks elsewhere. aaaand also i might be doing everything in Update now so that physics changes are deliberately limited to the framerate... this might not end up working how i want it to, actually.