Invalid framebuffer when opening model on MacOS X

The bug is triggered by launching the program followed by selecting Debug | Mock Model from the menu:

    CVF_ASSERT(validateProgram(oglContext));

on line 271 in `VisualizationModules/LibRender/cvfShaderProgram.cpp` fails and this aborts the program. The error log says:

    Validation Failed: Current draw framebuffer is invalid

This happens regardless of whether Use Shaders are selected or not in the Preferences.

If I comment out this line, the program apparently works fine, although I haven't done any extensive testing.

Upon startup, I get a bunch of:

    OGL(GL_INVALID_FRAMEBUFFER_OPERATION)

Drawing on hidden views is apparently no longer silently ignored from MacOS X 10.8, and this *may* be the cause of these messages. That is only speculation and I don't know if it is really related to the error, though.

Note: The string above is added in commit 0c0957f in #58.