Gamma correction fix for polylines

[lilleyse]

Some polyline shaders were missing gamma correction.

Fixes #7920

[cesium-concierge]

Thanks for the pull request @lilleyse!

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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.

[emackey]

GLSL changes look fine. @hpinkos I think you just need to test if these things look the right color (or at least, consistent colors) after this, like you did on the linked issue.

[hpinkos]

Thanks @emackey!

Yeah, looks good @lilleyse =)