fill_outline shader invokes undefined behavior

[kkaefer]

@jfirebaugh discovered in mapbox/mapbox-gl-native#8751 that we are using undefined behavior in the fill_outline shader:

mapbox-gl-js/src/shaders/fill_outline.fragment.glsl

Line 11 in c0dcee7

float alpha = smoothstep(1.0, 0.0, dist);

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.