Added IECoreHoudini.makeMainGLContextCurrent() python function.

This makes the OpenGL Context Houdini is using for the main window current. This allows Gaffer (in an upcoming pull request) to share OpenGL resources between the embedded Gaffer windows and the Houdini viewports.
Merge pull request #154 from johnhaddon/houdiniGLContextAccessor

Added IECoreHoudini.makeMainGLContextCurrent() python function.