Add a new alpha handling method, INPUT_PREMULTIPLIED_ALPHA_KEEP_BLANK.

Add a new alpha handling method, INPUT_PREMULTIPLIED_ALPHA_KEEP_BLANK.

This should fix most of the problems where you only process inputs
without alpha, but the framework still inserts an AlphaDivisionEffect
at the end because it loses track of the blank alpha state throughout
the pipeline and the output expects postmultiplied alpha.

There's one important case left, though: MixEffect will always reset
tha alpha state to premultiplied. We should probably fix that too
at some later stage.