Apply gamma correction to silhouette color

[lilleyse]

Fixes #7558

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

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

[emackey]

This change seems good, although the resulting color is still not exactly what the user requested. Looks like the color may be going through tone-mapping or similar, preventing the brightest colors from being reached.

Although small, this probably warrants a mention in CHANGES.md, since all silhouette colors will be changed by this.

[lilleyse]

@emackey Updated.

When scene.highDynamicRange = false the rendered color should be the exact requested color.