ofGLProgrammableRenderer fixes for RPi

These changes are only applied to RPi conditionals (uniqueShader) so this won't effect any other platforms.

This has been tested against and passes all the display errors/inconsistencies listed at 
 - allows shaders to be used in draw
 - stops GL INVALID_OPERATION at glDrawArrays when fbo/texture is used in update
 - consistent fbo/shader behavior in update() and draw()
passes all tests listed at
https://github.com/openframeworks/openFrameworks/issues/2593#issuecomment-26657402

I also did performance tests against non-fbo/shader app usage and saw no performance hits. This is where the previous fix failed (disabling the unique shader)

closes #2593  - I hope for good ;)