Gl points primitive colors

This fixes the constant shading of IECoreGL points primitives, which is separate to the per-point colour stuff in Hradec's pull request #25. Hradec's request uses the Cs shader parameter for passing per-point colours, which although it is in keeping with the current naming of parameters in the points primitive vertex shader, will actually conflict with the standard use of Cs for passing constant colours everywhere else in IECoreGL. So I'd like to get this fix in first before figuring out how to incorporate Hradec's work.

This request also contains one commit from #73, which was necessary for me to be able to test interactively in Maya.