modules/drizzle: add a mapped overlay style

this introduces a style= setting with values:
style=mask	simple alpha mask overlay
style=map	displacement mapped overlay

I might add a lighting option for the style=map mode with a
moving light source or something like that, but it's already
pretty slow as-is.

This is mostly just for more testing of the snapshotting, but
there's some interesting compositions enabled like:
module=compose,layers=submit:moire:drizzle

or just moire:drizzle, when style=map happens.

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