Preload images/audio on exp-lookit-images-audio page

[kimberscott]

Is your feature request related to a problem? Please describe.
Images on the exp-lookit-images-audio page can show up at slightly different times if one loads after the other.

Is your feature request related to a particular frame?
exp-lookit-images-audio

Describe your idea or solution
Preload images to be shown and don't display images until they have all loaded.

Describe alternatives you've considered
If images are supposed to be shown immediately, this doesn't incur an additional delay. But if they're supposed to be shown at a delay, do we want them to load first, then start the clock? Or load in parallel with any delay? The problem with loading in parallel is it gets complicated to preserve relative timing if an image isn't ready by the time it's supposed to be displayed.
Loading should "overlap" with any recording starting - i.e. the study is ready to start once recording is started if needed and images are loaded.

See https://lookit-mit.slack.com/archives/D01148G40UA/p1609768942000500 - thanks Avery Malone!

[kimberscott]

Added in v2.3.0! https://github.com/lookit/ember-lookit-frameplayer/releases/tag/v2.3.0

You can just update the code to the latest version to use these changes.