Only refresh scroll chrome when scroller is resized

[andydotxyz]

Progresses #5456

Checklist:

• Tests included.<- fewer refreshes, same test results :)
• Lint and formatter run with no errors.
• Tests all pass.

[dweymouth]

This is a good change (though haven't tested it yet) but just noting that it doesn't fix #5456 - the root cause there is the widgets themselves. e.g. here in Tree, it refreshes the entire content in the OffsetUpdated callback from the scroller: https://github.com/fyne-io/fyne/blob/develop/widget/tree.go#L491 (and similarly in table)

Also doing a full refresh on any resize here: https://github.com/fyne-io/fyne/blob/develop/widget/tree.go#L593

[coveralls]

coverage: 60.845% (+1.7%) from 59.108%
when pulling 8205f34 on andydotxyz:fix/5456
into 7bfcab6 on fyne-io:develop.

[andydotxyz]

It's a shame - in my testing this was night and day difference - using the split in fyne_demo is really laggy on develop and smooth with this PR applied.

Yes there is an "over refresh" in there still - but it's the lesser of the two. Maybe there are two different fixes to the issue reported as the resize and scroll refreshes do not have the same root cause?

[Jacalz]

Thanks. Looking really good. Just a small change I think you accidentally left in there (or due to being based on the wrong commit compared to develop).