Enables every combination of TextEdit and LayoutJob alignments (#7831)

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This is a fix/improvement that makes all kinds of alignments work in
`TextEdit` when a custom `LayoutJob` with halign is used.


I used the simplest approach possible to avoid unwanted bugs as I wasn't
sure what's safe to change, but there's potentially better ways to
achieve this. In particular, I'm not sure I fully understand the
rationale behind aligning rows in a `Galley` based on the latter's
leftmost border, considering the size is what's ultimately used in
widgets (in `TextEdit` at least).

Regardless, here's a demo of this PR:


https://github.com/user-attachments/assets/5d9801d7-73af-4576-80c5-47f169700462


* [x] I have followed the instructions in the PR template

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Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

tests/egui_tests/tests/snapshots/text_edit_halign.png

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