Move thickenSameSampleLines() into shader

[kwcantrell]

This is related to #190. Currently thick lines are computed on the cpu. However, this can be quite expensive on large trees (especially on the circular layout.

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It's not within the scope of this PR, but redrawing the entire tree whenever nodes are selected/unselected seems like it could get costly with really large trees. Do you think it'd be worth opening an issue for, later on down the line, making this more efficient?

(If you expect redrawing the tree to be a fast operation even for large trees then no worries. Just looking out for any potential future bottlenecks.)

Originally posted by @fedarko in https://github.com/biocore/empress/pull/212/files

[ElDeveloper]

Related to #190