Adjust line inset value for lines with `line-gap-width` in shader?

Steps to Trigger Behavior

1. Create a line style layer
2. Create another line style layer with a line-gap-width value that equals the line-width value of the first line style layer

Expected Behavior

There is no visible gap between the two lines

Actual Behavior

There is a small visible gap between the two lines:

The current shader behavior is to adjust the inset value of lines with a gapwidth to account for the antialiasing, but I think that this is causing the visible gap.

If you replace this line in the line vertex shader:
float inset = gapwidth + (gapwidth > 0.0 ? ANTIALIASING : 0.0);

with

float inset = gapwidth;

There is no longer a visible gap. This does make lines a bit visibly thicker when there isn't a corresponding line whose line-width equals its line-gap-width drawn on top of it. However, I think the main use case of the line-gap-width property is to use it with another line drawn on top of it, so I think it may be better to optimize for that use case. If there's a solution that would be optimized for both, that would be even better.

Here's what the current behavior looks like:

• Green lines have line-width: A
• Double maroon lines drawn under green lines have line-gap-width: A and line-width: B
• Double maroon lines not under green lines also have line-gap-width: A and line-width: B, for comparison
• Single maroon lines have line-width: B, for comparison

Here's what the altered behavior looks like:

We've used line-gap-width extensively in our core styles for a while, for roads, but I think the color contrast between the casing and the background color has always been so low that this issue wasn't really visible. We've recently been testing more high-contrast styling, and we are now finding that this is an issue. (I tested older version of mapbox-gl-js, I think this has probably always been consistent behavior).

Does this seem like something we can adjust?

/cc @aparlato

Start with src/shaders/line.vertex.glsl at the linked inset calculation and reproduce the two-layer line style described in the issue. Compare the current calculation with the proposed gapwidth-only calculation, including cases without an overlaid line. Done means eliminating the visible gap for matching line-width and line-gap-width values while assessing the reported standalone-line thickness tradeoff.