This helped me benchmark the atomic layout (#5830) changes.
I also realized that the label benchmark wasn't testing the painting,
since the buttons at some point will be placed outside the screen_rect,
meaning it won't be painted.
This fixes it by benching the label in a child ui.
The `label &str` benchmark went from 483 ns to 535 ns with these
changes.
EDIT:
I fixed another benchmark problem, since the benchmark would show the
same widget millions of times for a single frame, the WidgetRects
hashmap would get huge, causing each iteration to slow down a bit more
and causing the benchmark to have unreliable results.
With this the `label &str` benchmark went from 535ns to 298ns. Also the
`label format!` benchmark now takes almost the same time (302 ns).
Before, it was a lot slower since it reused the same Context which
already had millions of widget ids.
7d185acb41