Replace the fixed-size preview PNGs with a transparent webp master

Ship one transparent webp master per face. The store needs this master
to show a face without falling back to rendering it live. It replaces
the five presized opaque PNGs, which are no longer shipped because the
device sized previews are now auto generated on device by the launcher
preview renderer (AsteroidOS/asteroid-launcher#293).

Author: moWerk