Fix galley width calculation being off due to subpixel binning (#7972)

- fix for https://github.com/rerun-io/reality/pull/1075

The galleys row size was calculated by looking at the last glyphs pos_x,
which got changed to be rounded to integers when we added subpixel
binning. This introduced a subtle bug which caused the width of galleys
to be slightly off.
This PR fixes this by looking at the actual cursor position instead,
which is not rounded.

Also added a test to ensure this is correct. Previously, for the second
and last line, the `x` was too close to the `0`.

<img width="48" height="67" alt="image"
src="https://github.com/user-attachments/assets/a69a4cc3-b3f3-4553-ab92-73cb2e7a358c"
/>

---------

Co-authored-by: lucasmerlin <8009393+lucasmerlin@users.noreply.github.com>

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