Cue drawing and mirror reversal

[ghost]

1. I tried to optimise the cue drawing so that it isn't setting the colour on every frame, but it's drawing the cue for much shorter than letters. The bug is somewhere in oneFrameOfStim(), around the loop that checks n against cueDurFrames (that's what I changed)

2. Apparently you can mirror reverse psychopy stim by setting the size to a negative value. The size attribute of textstim doesn't seem to allow operations (stim.size *= -1 fails). Have a look at the classes for psychopy stim.