Fix rectangle that indicates the zoomed pixel area on hover being one pixel to small

[Wumpf]

What

• Fixes Image zoom rectangle is off by one pixel #3074

The outlines of the zoomed rectangle that we super-impose on the 2D space view were off by one pixel

Before:

Note how the rectangle on the picture is smaller than the zoomed-preview (zoomed out in this case)

After:

Note how the rectangle on the picture is identically to the zoomed-preview. The hover overlay hasn't changed, but a different pixel is hovered and the rect is correctly sized now

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

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