Maya GL Lighting Preview

This adds a lambertFragmentSource to IECoreGL::Shader and optionally uses it when lighting mode is enabled in Maya. Currently only the SceneShape is setup to use it.

Note the todo on line 82 of  src/IECoreMaya/DisplayStyle.cpp. I'm not sure its want we want in all cases, but currently this is the only use case, and it doesn't work without that forced behaviour.