Fix unexpected results for zoom-and-property function

[jfirebaugh]

        "fill-color": {
          "property": "property",
          "stops": [
            [{"zoom": 0, "value": 0}, "rgba(255,0,0,1)"],
            [{"zoom": 0, "value": 1}, "rgba(0,255,0,1)"],
            [{"zoom": 0, "value": 2}, "rgba(0,0,255,1)"],
            [{"zoom": 1, "value": 0}, "rgba(255,0,0,0)"],
            [{"zoom": 1, "value": 1}, "rgba(0,255,0,0)"],
            [{"zoom": 1, "value": 2}, "rgba(0,0,255,0)"]
          ]
        }

At z=0.1, I'd expect these fills to be rendered with opacity 0.9. Instead they are rendered with 0 opacity (fully transparent).

[lucaswoj]

I can imagine improving our handling of zoom-and-property functions by moving more interpolation logic to the shader via #pragmas

[lucaswoj]

Fixed by #2971

[jfirebaugh]

https://github.com/mapbox/mapbox-gl-test-suite/tree/master/render-tests/fill-opacity/zoom-and-property-function is still failing on gl-js master.