Fix normal calculation for CylinderGeometry

[felixpalmer]

The calculated normals of the curved side of the cylinder are wrong when the bottom and top radius are not equal, as the code assumes that the normal is always at right angles to the cylinder axis. This commit handles the more general case where the radii at the ends of the cylinder are not equal.

[cesium-concierge]

Thank you so much for the pull request @felixpalmer! I noticed this is your first pull request and I wanted to say welcome to the Cesium community!

The Pull Request Guidelines is a handy reference for making sure your PR gets accepted quickly, so make sure to skim that.

• ❌ Missing CONTRIBUTORS.md entry.
  • Please add yourself to the contributors file!
• ❌ Missing CLA.
  • Please send in a Contributor License Agreement (CLA) and comment back here to let us know to check this!
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

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[hpinkos]

Good catch @felixpalmer, thanks! I'll review this as soon as you send over your CLA. Instructions for doing so can be found in the cesium-consierge comment above

[hpinkos]

Thanks, we've received your CLA

[hpinkos]

Looks great! I can confirm the normals/tangents/bitangents are all in the right place for the sides of the cylinder now.

@felixpalmer can you add yourself to CONTRIBUTORS.md and add a note to CHANGES.md? After that this should be good to go. Thanks again!

[pjcozzi]

Thanks again @felixpalmer, we received your CLA.

[hpinkos]

Thanks again for the contribution @felixpalmer! I'll add a comment to CHANGES and CONTRIBUTORS in master