osgWidget OpenGL3.x compatibility issue

[gregkalata]

Currently osgWidget::Widget uses osg::PrimitiveSet::QUADS that is removed with GL3 https://github.com/openscenegraph/OpenSceneGraph/blob/master/src/osgWidget/Widget.cpp#L59 This could be replaced with osg::PrimitiveSet::TRIANGLE_STRIP using osg::DrawElementsUByte so that the vertex array can stay as is so change is quite localised. Can submit PR if that approach is acceptable.