IECoreGL::Selector : Fix leak of glClearColor and glClearDepth
IECoreGL::Selector : Fix leak of glClearColor and glClearDepth

This was exposing itself in Maya 2016 where the viewport background is no longer a gradient, but rather a solid color.

appleseedhq/cortex,appleseedhq/cortex,hradec/cortex,appleseedhq/cortex,hradec/cortex,hradec/cortex