Bug fixes:

- Fix the usage of VideoEngine::render
- Fix the behaviour when closing a viewer tab or deleting a viewer
node: either cases a undo action is stacked.
- Histograms can now be created from the "Manage layout" buttons of
every panes
Merge branch 'workshop' of https://github.com/MrKepzie/Natron into workshop

Conflicts:
	Gui/Histogram.cpp