Point cloud styling fixes

[lilleyse]

${feature.propertyName} and ${feature['propertyName']} syntax now works for point clouds using the GLSL styling backend. It was an oversight in getShaderExpression and required just a few small changes.

The bigger change here is supporting feature properties with non-alphanumeric characters, like ${feature['temperature value 😀']}. The shader was using the property name as-is which doesn't compile if the name has spaces or unicode characters. Now the shader attribute is named with a unique id.

Local sandcastle

[cesium-concierge]

Thanks for the pull request @lilleyse!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

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

[mramato]

Awesome! Thanks for the quick fix @lilleyse!

[sanjeetsuhag]

@lilleyse It would be good to have tests for handling properties with spaces and Unicode characters

[lilleyse]

Added a test and confirmed the test fails in master. @sanjeetsuhag do you have other feedback?

[mramato]

Would love to see this go into the next release. @sanjeetsuhag is there anything else you're looking for here?

[sanjeetsuhag]

Code, sandcastle and test look good. Thanks @lilleyse!