Get notified when image is rendered

[AmadeusW]

The goal is to hide the picture until it is fully downloaded and rendered.
In theory, A-Frame exposes materialtextureloaded that can be used to achieve this, but in practice this event wasn't fired, even if I updated the material like so:
document.getElementById("leftPlane").setAttribute("material", "src: url(...)")

A temporary workaround is to black out the screen for 150ms which works well for already viewed pictures, but is inadequate for newly downloaded photos (seems like web browser cache is not used)

See imageLoader.js