ProjectGuiSerialization: signal a bug
use inline toRGBA functions
ViewerInstance: begin multithreading cleanup. Not all warnings correspond to potential errors.
ViewerInstance: use bufferAllocated instead of mustFreeBuffer + add a few warnings/questions
ViewerInstance: mark more dead code
Merge branch 'workshop' of https://github.com/MrKepzie/Natron into workshop

Conflicts:
	Engine/ViewerInstance.cpp
	Global/GitVersion.h
ViewerInstance: more questions + remove unused member