modules/mixer: add a fade style, make default

This is not optimized at all and tends to hurt the FPS
significantly.  This is one of those things that would hugely
benefit from SIMD, but even without SIMD it could be done better.
I just slapped together something obvious for now, as I'd like to
focus more on the rkt side but need a better fader for scene
transitions than style=flicker.

Also changed {a,b}_module= preferred values to blank,compose
so you see something happen if you just run --defaults.
Otherwise, the compose,compose would just fade between two
identical compositions invisibly.

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