Avoid firing 'load' or 'error' events for <img> when the loaded image didn't change

[zcorpan]

Currently, in the "update the image data" algorithm for img, if the loaded image is the same as the previous image, it will still fire a load event. Similarly for error (see #4480).

Investigate if it's compatible to change this to avoid firing load or error when there was no change (other than updating the current pixel density).

See discussion in #8008

@zcorpan

If this change appears too risky, I think we could first try to change the “load the image data” algorithm to avoid firing “load” and “error” events in more cases. If that is compatible, then it should alleviate the main compat risk for auto-sizes.

@yoavweiss

At the same time, I kinda like the option of avoiding firing the "load"/"error" events in cases where the image didn't actually loaded.