IECoreGL Selection Fixes

This makes a couple of fixes to IECoreGL's selection code, addressing https://github.com/ImageEngine/gaffer/issues/53 and https://github.com/ImageEngine/gaffer/issues/160. The problems don't show themselves at all on my Mac, because it falls back to GL_SELECT mode for selection, so I've been testing on my workstation over VNC. The unit tests all pass there and I've also manually verified the problem as fixed in Gaffer, but the latency is too great to do much more testing - it might be a good idea to have a couple of developers run this for a few days before it gets pushed to production.

I'm also a little curious as to how the "primitive:selectable" attribute was working at all before - it kindof implies that we were using GL_SELECT selection mode everywhere where it was needed. That mode is very slow on the workstations so it might be worth a little bit of investigating...