simplify & fix smoothing

set SURFACE to null in setMode to force a true re-initialization in any
case even if called multiple times

and don't _smooth _noSmooth in display because Surface already does
that on its own.
add gamejs.util.prng static functions

one generator instance per gamejs instance; also fixed the weird
alea constructor
Merge git://github.com/oberhamsi/gamejs
Merge branch 'master' of https://github.com/domasx2/gamejs