Added pixel ratio to czm_metersPerPixel

[IanLilleyT]

After closing #8113 we noticed that "Size billboard in meters" inside this sandcastle looked different at varying resolution scales. Turns out I missed the czm_metersPerPixel function which is used by billboards, clipping planes, and ground polylines. So now czm_metersPerPixel handles the pixel ratio multiply and these shaders no longer do their own czm_pixelRatio multiply.

I confirmed that polylines, billboards, and clipping planes work consistently between resolution scales

[cesium-concierge]

Thanks for the pull request @IanLilleyT!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
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• ❔ Unit tests were not updated.
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Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[lilleyse]

Thanks @IanLilleyT