Mask modifier for sequences

This modifier uses a mask set in the modifier settings
and multiplies strip by it. Alpha channel will also be
multiplied by mask, which makes it easy to mask some
objects on footage and alpha-over them in sequencer.

Actually, this modifier sets alpha for byte strips
directly (since byte is always straight alpha) and
multiplies float buffer by mask (flaots are premulled)
so in both cases masked strip could be easy alpha-overed
without any artifacts.

It uses own structure with only SequenceModifierData
property in to preserve both forward and backwards
compatibilities (using new structure ensures modifier
will be ignored on load in older blenders, the same
happens for mesh modifiers actually).

Request from Pablo Vazquez.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56582 954f8c5b-7b00-dc11-b283-0030488c597c