Fixed OpenGL drawing when embedded in Houdini.

Because IECoreGL is used in both the Houdini viewport and the Gaffer GLWidgets, we needed to make sure that the GL context in each shared resources.

This relies upon https://github.com/ImageEngine/cortex/pull/154.
Merge pull request #618 from johnhaddon/houdiniGLFix

Fixed OpenGL drawing when embedded in Houdini.