We invoked the `extract_default_ui_camera_view` system twice: once for
2D cameras and once for 3D cameras. This was fine before moving to
resources for render phases, but, after the move to resources, the first
thing such systems do is to clear out all the entities-to-be-rendered
from the previous frame. So, if the scheduler happened to run
`extract_default_ui_camera_view::<Camera2d>` first, then all the UI
elements that it queued would be overwritten by the
`extract_default_ui_camera_view::<Camera3d>` system, or vice versa. The
ordering dependence is the reason why this problem was intermittent.
This commit fixes the problem by merging the two systems into one
systems, using an `Or` query filter.
## Migration Guide
* The `bevy_ui::render::extract_default_ui_camera_view` system is no
longer parameterized over the specific type of camera and is hard-wired
to either `Camera2d` or `Camera3d` components.
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