Floating effect now uses animate() instead of pixel_y while loop thingy.

Distinct actions that indefinitely change a mob's pixel_y or pixel_x, like strapping someone to a roller bed, should also alter new variables old_y or old_x accordingly. This means that floating and jittering animations no longer interfere, as the animations use old_x/old_y as the "base" position.

Entering areas with gravity from areas without grabity now removes floating effect.
gravitychange() now makes all affected mobs have the floating animation unless magbooted, etc.

New mob proc Check_Dense_Object() seperated from Process_Spacemove(), checks for adjacent objects or turfs that can be pushed off, also handles magboots on gravity-less floors.