Fix unexpected results for fill-opacity property function

[jfirebaugh]

        "fill-opacity": {
          "property": "property",
          "stops": [
            [
              0,
              1
            ],
            [
              4,
              0
            ]
          ]
        }

For values 0, 1, and 2, I'd expect the rendering to look like:

But instead it renders everything with opacity 0.

Possibly related to #2827.

[lucaswoj]

Fixed by #2971

[jfirebaugh]

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