`bevy_ui_render`: Possibly deduplicate code in various prepare_* functions

> Added fix to other pipelines as well. Feels like a lot of duplicated code though. I wonder if some parts could be consolidated.
> _Originally posted by @GiantBlargg in https://github.com/bevyengine/bevy/issues/22922#issuecomment-3894160226_

Investigate whether code can be deduplicated in prepare_shadows, prepare_gradient, prepare_ui_nodes, prepare_ui_material_nodes, and prepare_ui_slices. It’s unfortunate that the fix had to be applied to five different places for the referenced issue