Performance improvements

[mhturner]

Performance improvements are needed for layered, multi-component stims. Issue can cause frame drops and loss of timing accuracy for complex stimuli.

Performance issues seem to be especially obvious when calling multiple, distinct shaders. Maybe share a shader for non-textured objects and pass all vertices as concatenated list - e.g. add_stim method or similar to add stimuli to a shared shader that is only spun up once. Harder to see how to get around this with multiple textured stims

[mhturner]

Big part of this issue seemed to stem from re-creating the stim object from scratch on each paint call in order to change its properties as a function of time. See partial fix here for most stims that uses existing stim object and methods to change properties or template stim objects. Still need to optimize stims MovingSpot, MovingPatch with a method to change object size