GPUTypeFromToGL: Support glObjType 3 - Currently bound FBO.

Reading/Writing the currently bound FBO resp. its 1st color attachment,
allows to directly use this functionality from within processing chains
/ slots of the imaging pipeline, either as part of preflip/postflip
operations, or as part of Screen('TransformTexture').
testGLInterop: Revert to default type 0 test case.