Taffy's `round_layout` uses a rounding function that rounds away from zero, so x bounds from -100.5 to 99.5 (width 100) will be rounded to -101 to 101 (width 101), gaining a pixel.
This commit disables Taffy's builtin rounding with a function that round halfs up rather than rounding them towards zero.

Taffy's builtin rounding is disabled and rounding is now performed by `ui_layout_system`.

Instead of rounding the values of the internal layout tree as Taffy's builtin rounding does, we leave those values unmodified and only the values stored in the components are rounded. This requires that we walk the tree for the uinode geometry update rather than iterating through a query.

Because the component values are regenerated each update, that should mean that UI updates are idempotent now and make the growing node behaviour seen in issue bevyengine#8911 impossible.

Added two helper functions `round_ties_up` and `round_layout_coordinates`.

* Added comments explaining the behaviour of the `round_ties_up` function.
* Changed the first predicate to use `<=` instead of `<`. This doesn't change the function's behaviour (since `(0.).ceil() == 0.`) but it feels like it makes it a little easier to undertand.