Optimize line drawing by avoiding per-tile work

[mourner]

This is a slightly messy attempt at winning back some of the performance that regressed with #3342 back, until we solve the challenge of ensuring all tiles are always rendered with the same style.

The idea is to simply check whether the program changed after useProgram call, and not do a big portion of the drawing work if it didn't.

On my 3342 benchmark, results are as follows:

• v0.19: 8.3ms
• v0.25: 17.2ms
• master: 15.3ms (looks like some of your recent changes saved us some ms @jfirebaugh!)
• this PR: 12.2ms 11.7ms

👀 @lucaswoj @jfirebaugh

[jfirebaugh]

I'd prefer to follow the optimization that native uses: each program object records the value which was last set for each uniform, and avoids calling glUniform* when the value isn't changing. This optimization benefits every layer type as well as the k === 0 case here (you can reuse the uniform values set by a previous layer, if they're not changing).

[mourner]

@jfirebaugh yes, but that sounds like a more involved refactor (#145). Meanwhile this PR is an easy short-term stopgap that we can revert at any time.

[jfirebaugh]

Fair point!

[mourner]

@jfirebaugh added another commit that removes redundant state changes for pattern/dash-related line drawing code by tracking if the tile zoom changed between tiles. Also cleaned it up a bit. This brings the benchmark result a little further down, from 12.2 to 11.7.

[mourner]

frame-duration benchmark result isn't as significant because of the averaging I mentioned in #3342, but still noticeable:

benchmark	master	optimize-line-draw
frame-duration	9.6 ms, 5% > 16ms	8.9 ms, 1% > 16ms