Fix pixel alignment math

[bryankeller]

Details

This fixes a subtle screen-pixel alignment bug. The existing math causes slight oscillations between 2 adjacent pixels, which can cause some additional layout churn.

Related Issue

N/A

Motivation and Context

Bug fix / optimization

How Has This Been Tested

Types of changes

• [ ] Docs change / refactoring / dependency upgrade
• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.