ges-smart-mixer / ges-uri-source: add a framepositionner element which

... adds metadata on the video buffers and parse them in the smart-mixer.
	+ This is done because we want to keep positionning properties
	+ and set them on the dynamic mixer pad.

Conflicts:
	ges/Makefile.am
smart-mixer / uri-ssource : update z order according to layer priority.