Fixed dirty propagation of Shaders through ShaderAssignments.

This currently only works for RenderManShaders and OpenGLShaders - see todo explaining the problem with ArnoldShaders.
Removing workaround for 3delight bug, which was fixed in 10.0.138.
Added rudimentary shader updates to InteractiveRender.

It needs to be much smarter about what to update and when, but it's functional for small scenes.