Supporting click-selection for GU_CortexPrimitives

Thanks to John for his glProgram tomhackery.
Preventing popping during click-selection.

Its not clear why, but hovering on GU_CortexPrimitives during GR_RENDER_OBJECT_PICK mode seems to flip the mode to GR_RENDER_MATTE. Since we're not supporting that on its own, we consider it a continued pick for now.
Highlighting selected GU_CortexPrimitives
Merge pull request #138 from andrewkaufman/clickSelect

Click Selection of GR_CortexPrimitives