Closes FR-133

`cosmic-text`'s BiDi line shaping machinery (`ShapeLine::new`) asserts
that every BiDi paragraph is following the same direction. It turns out
that our `gpui_wgpu` line shaping adapter would not account for that
leading to panics if text contained multiple BiDi paragraphs with
multiple directions. Not super common but worth fixing as the fix is not
super complicated. At the same time, I'd like to point out that there is
a patch submitted to `cosmic-text` that redoes shaping logic so that
this is no longer illegal:
https://github.com/pop-os/cosmic-text/pull/508 If it gets accepted, we
can revert this fix.

Release Notes:
- Fixed panic in `gpui_wgpu` if text contained multiple BiDi paragraphs
with mismatched directions.