compositor: refactor in order to have alpha sources blended over non alpha ones

Submitted by Jean-Michel Hautbois (jmleo)

Link to original bug (#751484)

Description

The idea is to be able to compose a YUV with a AYUV into a YUV without having to convert twice.

It could be useful to use porter/duff operations for this.
Needs some refactoring in compositor, in order to select the proper blend function based on sources opacities.