compositor: refactor in order to have alpha sources blended over non alpha ones
Submitted by Jean-Michel Hautbois (jmleo)
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.