BF: GratingStim with shaders==False was leaving textures attached
ENH: basic rendering using FBO now working inside visual.Window
ENH: speed-up by removing all use of GL_DEPTH_BUFFER (and clearing it)

When using the FBO clearing the depth buffer slows things down, and
depth testing is disabled anyway right now
ENH: Although rendering via an FBO is working but turned off by def (for speed)

also minor amendment to timeByFrames
BF: Experiment.getComponentFromName had a param.val error