moving selector examples in docs #647

I just got an idea, when we eventually make emulated events for testing we can also use it to make make selectors move in the examples gallery. Should figure out a way to do it such that the code for emulating the events doesn't show up in the example code 🤔 . Simple thing that works for linear and linear region is if we modify the pygfx gallery scraper to make a video that goes through the min and max limits of the selector over ~50 frames or something. Harder to think of for rectangle and (future) polygon selectors. Maybe a predefined motion? We could add it to the sphinx_gallery_pygfx_docs = '...'.