BF: Fix display list leaks in stimuli objects

The OpenGL display lists created by four of the stimuli objects were not
deleted. In most experiments this may not matter because the stimuli are
usually reused instead of being deleted and created again. Still this is
an improvement because it allows PsychoPy to scale better in such cases.
Merge pull request #425 from philippwiesemann/fix-stimuli-displaylist

BF: Fix display list leaks in stimuli objects