Remove camera from UiBatch

[ickshonpe]

Objective

A TransparentUI phase's items all target the same camera so there is no need to store the current camera entity in UiBatch and ending the current UiBatch on camera changes is pointless as the camera doesn't change.

Solution

Remove the camera fields from UiBatch, UiShadowsBatch and UiTextureSliceBatch.
Remove the camera changed check from prepare_uinodes.

Testing

The multiple_windows and split_screen examples both render UI elements to multiple cameras and can be used to test these changes.

The UI material plugin already didn't store the camera entity per batch and worked fine without it.