[Fiber] use srcset to trigger load even on img mount (#30351)

In https://github.com/facebook/react/pull/23316 we fixed a bug where
onload events were missed if they happened too early. This update adds
support for srcset to retrigger the load event. Firefox unfortunately
does not trigger a load even when you assign srcset so this won't work
in every browser when you use srcset without src however it does close a
gap in chrome at least