No load or error event for <video poster>

[zcorpan]

While writing tests for #8428 I needed to use ResizeObserver to detect when the poster image is loaded, because there are no events fired for the poster image being loaded or failing to load.

If you specify the correct dimensions with <video width height> then it's not possible to use ResizeObserver, either.

Would it be useful for web developers to have events here? e.g. posterload and postererror.

cc @whatwg/media

[jdm]

This is also helpful for writing tests that verify that the poster image is affected by things like CSP, mixed content, etc. Servo has event handlers that are test-only to work around this.