bind defaultShader in setup() for RPi/GLProgrammable

This fixes an issue in the Programmable GL Renderer where RPi would not have the default shader bound when a drawing call was issued in update().

Fixes issue #2593

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
Merge pull request #2597 from tgfrerer/fix-RPi-ProgrammableGL-defaultshader

bind defaultShader in setup() for RPi/GLProgrammable