GaussianMask parameters can't be changed after construction

The drifting grating plugin's GaussianMask class takes two parameter variables (std_dev and mean). However, rather than holding on to the variable objects, the constructor dereferences them and uses their current values to construct the mask. This means that, contrary to user expectations, later changes to the variables have no affect on the stimulus.

The GaussianMask class should be updated so that changes to the parameter variables affect the appearance of the stimulus.