Sometimes when moving a window, having a tooltip attached to the mouse
pointer, or scrolling a `ScrollArea`, you would see this disturbing
effect:

This is caused by us rounding many visual elements (lines, rectangles,
text, …) to physical pixels in order to keep them sharp. If the
window/tooltip itself is not rounded to a physical pixel, then you can
get this behavior.
So from now on the position of all
areas/windows/tooltips/popups/ScrollArea gets rounded to the closes
pixel.
* Unlocked by https://github.com/emilk/egui/pull/7709
This adds a new mode, `UPDATE_SNAPSHOTS=force`, which will lower the
threshold to zero, overwriting every image that is not _exactly_ the
same.
Most comparisons has a threshold because different GPUs render slightly
differently. However, setting that threshold accurately can be hard.
Sometimes a test will pass locally, but fail on CI. In those cases you
want to force an update of the failing test. You can use
`UPDATE_SNAPSHOTS=force` for that.
And sometimes a small change _should_ update all images, but the change
is so tiny that it falls under the threshold. Still, you want to make a
point of showing that these images have changes. You can use
`UPDATE_SNAPSHOTS=force` for that.