Added a framebuffer to the default GL context on OS X.
Added a framebuffer to the default GL context on OS X.

This is similar to the pBuffer created on Linux, and fixes several of the Selection.py test cases.

goddardl/cortex,hradec/cortex,hradec/cortex,lento/cortex,goddardl/cortex,hradec/cortex,davidsminor/cortex,goddardl/cortex,davidsminor/cortex,appleseedhq/cortex,lento/cortex,davidsminor/cortex,appleseedhq/cortex,davidsminor/cortex,appleseedhq/cortex,lento/cortex