[osg-submissions] Selectively ignoring handled events

[Ole-Morten Duesund]

Robert Osfield wrote:
> Hi OM,
> 
> The event mask would be an interesting approach that be worth
> experimenting with.  I don't have the available time to go try this
> out, so could you have a tinker and see how you get on.

Ok, turns out this was even easier than expected.
This will break any code that depends on the absolute values of 
osgGA::GUIEvent::EventType, but I'd consider that kind of code broken 
anyway.

This changes osgGA::GUIEvent::EventType so I can use them in a bitmask. 
I also added the ability to set/get the mask in osgGA::GUIEventHandler 
and changed osgViewer::Viewer to consider the mask when delivering 
handled events to cameraManipulators. The same could easily be done for 
the generic case of eventHandlers if that should be of interest.

This will let the cameraManipulators signal to the osgViewer::Viewer 
which handled events they're willing to ignore.

Simplest testcase I could come up with was to use the 
NodeTrackerManipulator and setting the mask to 
osgGA::GUIEventHandler::DRAG while having an eventhandler that returns 
true for DRAG events. This basically stops the NodeTrackManipulator from 
working. Setting the mask to NONE (which is the default) brings back the 
normal behaviour.

- OM
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