Fix QImage's step size in ImageDisplay plugin

[iche033]

🦟 Bug fix

Fixes https://github.com/gazebosim/gz-rendering/issues/945

Summary

The QImage constructor that was used expects the images are 32 bit aligned. This is not true for our RGB_INT8 formats. (24 bits) The PR fixes the issue by explicitly specifying the bytesPerLine field in the constructor.

Tested with camera_sensor.sdf world by changing the image width to be 321 (from 320)

Before

After

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[iche033]

gz_gui-ci-pr_any-focal-amd64 test failures look unrelated.