Fix fill-color property-function interpolation of alpha value

[peterqliu]

mapbox-gl-js version: v0.20.1

Steps to Trigger Behavior

Check out https://jsfiddle.net/p6tjyehh/. DDS fill-color based on the property someNumber.

Expected Behavior

I expect the polygon to be nearly transparent here, since the styling rule is [[0, 'rgba(30,60,90,0)'], [1,'rgba(30,60,90,1)']], and the value I provided is near 0.

Actual Behavior

The polygon is opaque. There's some quirk with the alpha value 1, though; when I change it to 0.99 (making the styling rule [[0, 'rgba(30,60,90,0)'], [1,'rgba(30,60,90,0.99)']], it works properly

[jfirebaugh]

This, #2827 and #2828 are probably all related.

[mollymerp]

Just tested the example and this is not an issue with circle-color interpolation, so for now it seems isolated to the fill-color property functions

https://jsfiddle.net/LLgL3byL/

[lucaswoj]

This bug is caused by draw_fill's inability to understand property functions when deciding whether to render in the opaque or translucent pass..

For this reason, we should consider StyleLayer#paint and StyleLayer#layout deprecated and work towards #2739.

Like #2065, this use case would benefit from some methods to determine the range of a style function.