Rename "text-halo" to "text-stroke"

[lucaswoj]

From @lucaswoj on December 23, 2016 14:36

See the discussion here: mapbox/mapbox-gl-style-spec#379

Copied from original issue: mapbox/mapbox-gl-style-spec#636

[lucaswoj]

From @ajashton on January 2, 2017 19:16

The functionality of the current text-halo is much closer to CSS's text-shadow than text-stroke. The halo is only ever outset from the symbol, whereas a stroke is centered directly on the outline of the symbol. The halo can be blurred while strokes cannot. Strokes can have various dasharrays, round/bevel/miter joins, and offsets but halos cannot.

Going with 'stroke' for the circle symbol as discussed in #379 seems reasonable, but I think the situation for text is quite different and it might not be worth conflating the two, unless the plan is to bring halo and stroke features more in line with each other?

[lucaswoj]

From @nickidlugash on January 3, 2017 1:42

Agree with @ajashton on all the above points.

The functionality of the current text-halo is much closer to CSS's text-shadow than text-stroke.

And using a Photoshop analogy, the current text-halo is much closer to the outer-glow blend mode than the stroke blend mode.

[jfirebaugh]

Closing here; see #6584.