visual/VisualStim: Enable pixi updates when setting opacity

[thewhodidthis]

@apitiot @tpronk Tweak the onChange() callback for opacity to activate this._needPixiUpdate so that dynamically altering its value is picked up when _updateIfNeeded() gets called. Note also that ShapeStim's constructor needs no separate this._setAttribute() for opacity, closes #202 and closes #275

[apitiot]

I am not sure that this is necessary.

The mistake I made was to have _onChange(false, false) instead of _onChange(true, false) in ShapeStim.js since it is necessary to recreate the PIXI object when the opacity changes for a ShapeStim. However, I believe we do not need it systematically. For instance, changing the opacity of the MovieStim does not require a new PIXI object.

Shall we go over the stimuli and check when it is needed and when it is not? Let us try not to create/recreate PIXI objects when not necessary.

Alain

[thewhodidthis]

@apitiot Having the tweak applied inside ShapeStim's constructor, not VisualStim's in general was the first thing I tried. Seems to be having no effect, you can visit the project below to confirm. Could it be something else is getting in the way? I have checked that opacity is kept off the super() call as a way to make sure the this._addAttribute() is respected?

run.pavlovia.org/thewhodidthis/rule_learning_fall2020_num_spa/html →