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.
Fixed ParameterisedHolder parameterChanged() crashes.

These occurred if the parameterChanged() method left a parameter with an invalid value and then raised an exception. ~ParameterModificationContext would then raise another exception during exception handling and boom. See comments in ParameterisedHolder.cpp for more details.
Merge pull request #619 from johnhaddon/parameterChangedExceptionHandling

Fixed ParameterisedHolder parameterChanged() crashes.
Fixed GLSL shaders used by the UI for OpenGL 2.1.

This relies on the recent Cortex changes for OpenGL 2.1 compatibility in IECoreGL.
Merge pull request #620 from johnhaddon/gl2

Fixed GLSL shaders used by the UI for OpenGL 2.1.