Fix typo when cloning ShaderSource.

[bagnell]

This might fix any bugs with picking translucent geometry. I quickly looked at existing issues but didn't find anything.

[cesium-concierge]

@bagnell, thanks for the pull request! Maintainers, we have a signed CLA from @bagnell, so you can review this at any time.

⚠️ I noticed that CHANGES.md has not been updated. If this change updates the public API in any way, fixes a bug, or makes any non-trivial update, please add a bullet point to CHANGES.md and comment on this pull request so we know it was updated. For more info, see the Pull Request Guidelines.

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[mramato]

I think someone on the forum recently had translucent picking issues @hpinkos or @ggetz may know for sure.

[ggetz]

@mramato Were you talking about this post? https://groups.google.com/forum/#!searchin/cesium-dev/pick%7Csort:date/cesium-dev/C-yXSf6-8aQ/LW5ZZIMHBwAJ

[mramato]

Possibly, I just remember hearing something about a translucent picking bug recently.

[hpinkos]

Good catch @bagnell!

[cesium-concierge]

Congratulations on closing the issue! I found these Cesium forum links in the comments above:

https://groups.google.com/forum/#!searchin/cesium-dev/pick%7Csort

If this issue affects any of these threads, please post a comment like the following:

The issue at #6234 has just been closed and may resolve your issue. Look for the change in the next stable release of Cesium or get it now in the master branch on GitHub https://github.com/AnalyticalGraphicsInc/cesium.

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