Hack transition from undefined fill-outline-color

[anandthakker]

Closes #3657

Using "option 1" described here

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

[anandthakker]

This is an unfortunate use of private method _calculateTargetValue, but my feeling is that it's (slightly) less bad than the only other non-invasive alternative that I could see, which would be to hack interpolate.color so that it can handle undefined values for from. I prefer the hack here because it's more localized, and will be easier to excise when we get rid of fill-outline-color.

[anandthakker]

benchmark	master 2793b42	3657 193e124
map-load	101 ms	139 ms
style-load	91 ms	87 ms
buffer	943 ms	954 ms
fps	60 fps	60 fps
frame-duration	4.7 ms, 0% > 16ms	4.6 ms, 0% > 16ms
query-point	0.67 ms	0.79 ms
query-box	59.62 ms	62.67 ms
geojson-setdata-small	6 ms	5 ms
geojson-setdata-large	113 ms	172 ms

[anandthakker]

@mourner @lucaswoj ready for 👀

[anandthakker]

Transcribing from chat

@anandthakker I agree with you that using _calculateTargetValue feels non-optimal
Do you think it’d be possible to fix the bug by changing the construction of fill-outline-color‘s StyleTransition object?
Another alternative:
tidy up the StyleTransition API and get rid of the underscore

We agreed to go with the second option, but after looking into this (and also, alas, a detour 👇🐰🕳 investigating the first option), I'm now 👎 on removing the underscore. The method we'd have to remove the underscore on, _calculateTargetValue, really is an internal implementation detail of StyleTransition, and I don't think there's any natural/legit part of it to extract into a public method. Doing so feels contrived, which IMO is no better than actually using the underscored method in the first place.

All that said, in 20ea68f i did revise to a slightly cleaner approach by relying on the public--though sorta deep--object hierarchy rather than the private method

StyleTransition --> StyleDeclaration --> value
  +- oldTransition: StyleTransition // forms a chain of ongoing transitions
  +- declaration: StyleDeclaration

StyleDeclaration
  +- value: ... // the declared value, which might be undefined

cc @lucaswoj @mourner

[lucaswoj]