Single-pass halo text

[d1manson]

With this change, halo text is rendered in the same shader pass as the text itself.

The motivation for this was to improve the appearance of overlapping text as shown below:

Presumably it also has a somewhat beneficial impact on performance.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs ...none in this case
• post benchmark scores
• manually test the debug page

[d1manson]

I have just noticed a potential issue, which is that now the halo from one character can overlap with the non-halo part of the previous character (within the same piece of text). The text-letter-spacing style value can be increased as a hack workaround, but it's not really what I wanted.

[kkaefer]

refs #596

[lbud]

Re: #4717 (comment) — right, we ran into that issue in #596. As @jfirebaugh noted in #4709 (comment),

The current behavior was chosen because it's almost always the desired result for text symbols. If we were to implement the alternative behavior, we would need to take care to preserve the current rendering of text. I can think of a few approaches:

• Make halo overdrawing happen for icons, but not for text
• Make halo overdrawing an opt-into behavior based on a style property
• Make halo overdrawing happen for separate features, but preserve the current behavior for individual icons/glyphs associated with a single feature

We won't be able to merge this PR as-is as it would negatively impact label rendering for a lot of existing map styles. It's also clear from your intention here that option 1 from the above comment isn't necessarily the right solution for all users, which is helpful to know. Let's centralize discussion of how this should be solved in #4709.

[ChrisLoer]

If we handle the other considerations that are blocking this, there's one more thing to worry about.. we're actually planning to rely on halos being done in a separate draw call on gl-native: mapbox/mapbox-gl-native#9009. The reason is that device compatibility limitations require us to have 8 or fewer vertex attributes (and using two draw calls allows us to selectively use only fill_color or halo_color). Even though that requirement doesn't directly affect gl-js, we want to keep the shader implementations in sync between the two code bases.