bootanim: Allow configurable rescaling

Some themes carry bootanims on specific resolutions that look like
crap on different screen densities. We can have scale those into
the correct size, now.

RM-290

Change-Id: Ib8ed04899dd46fed33b15d08fce4f7411b925f0c
TODO: Plug a sliding scale (and auto-adjust into the theme engine?)