Bug fixes:

- A disabled combo box will have its label set black
- Any disabled knob will have its label set black
- It is now possible to disable group parameters (see OFX KitchenSink)
- Fixed a bug in the VideoEngine where the hash computation wasn't done
when we were changing the tree.
- Fixed the null render format (it wasn't initialised always)
- A cached image whose RoD is the project format will have its RoD
checked against the current project format before considering it as a
valid image.
Git version & update OpenFX
Merge branch 'workshop' of https://github.com/MrKepzie/Natron into workshop

Conflicts:
	Engine/EffectInstance.cpp
	Engine/EffectInstance.h
	Engine/Node.cpp
	Engine/Node.h
	Engine/OfxEffectInstance.cpp
	Engine/OfxParamInstance.cpp
	Engine/ViewerInstance.cpp
	Gui/ViewerGL.cpp