Fix outline rendering

[kkaefer]

@jfirebaugh discovered in #8751 that we are using undefined behavior in the fill_outline shader: https://github.com/mapbox/mapbox-gl-js/blob/c0dcee76ed5c7d0559bf271ed85692b1b81c654c/src/shaders/fill_outline.fragment.glsl#L11

It seems that some implementations correctly interpolate between 1.0 and 0.0 based on the t value, but per the spec, it's undefined behavior.

It also fixes outline rendering, which has regressed at some point.

Current behavior when drawing an outline with a color different from the fill color:

New (and correct) behavior:

We are drawing outlines as antialiased GL_LINES around the polygon. When the color is identical to the fill color, we are clipping the outline at the polygon bounds (see first image), but we can't do that when the color is different. Therefore, we either render the fill below or above the line. At some point, this regressed into rendering it always below (and in this case, it's clipped/covered by the fill).

[friedbunny]

This fix looks significant enough to warrant changelog entries (at least, for iOS). 🙇

[jfirebaugh]

Working on the failing query test in mapbox/mapbox-gl-js#4604.

[jfirebaugh]

@kkaefer Even with these changes, macosapp still isn't rendering correctly for me locally.

[kkaefer]

@jfirebaugh What GPU are you using locally? What's the maximum line width in your OpenGL implementation?

[jfirebaugh]

Intel Iris Graphics 6100, GL_ALIASED_LINE_WIDTH_RANGE = [1, 7], GL_SMOOTH_LINE_WIDTH_RANGE = [0, 7]

[kkaefer]

Fixed in #9699