Fixes https://github.com/bevyengine/bevy/issues/14277.
May also fix https://github.com/bevyengine/bevy/issues/14255, needs
verification.
Explicitly order `CameraUpdateSystem` before `UiSystem::Prepare`, so
that when the window resizes, `camera_system` will update the `Camera`'s
viewport size before `ui_layout_system` also reacts to the window resize
and tries to read the new `Camera` viewport size to set UI node sizes
accordingly.
I tested that explicitly ordering `CameraUpdateSystem` _after_ triggers
the buggy behavior, and explicitly ordering it _before_ does not trigger
the buggy behavior or crash the app (which also demonstrates that the
system sets are ambiguous).
---
`CameraUpdateSystem` is now explicitly ordered before
`UiSystem::Prepare` instead of being ambiguous with it.
216dba23cb