Blender Git Loki

Bug fix #2104

When using Sequencer for Alpha-Over images, it was assuming "Key" alpha by
default, and converting images to "Premul" (= multiplying RGB with alpha).
This isn't very nice, also because alpha generated by MBlur in Blender
always is of type Premul already.

Solution; make this an option. In the Nkey panel for strips you can set
or disable Premul now. As extra also added the "Filter" and "Mul" options,
as already there for Movie strips. Can easily be extended to more options
for various postprocess effects.