Functions for layout properties

[mourner]

Currently bucket rules can only be static, which means attributes important for point and label placement (fontSize, padding, size, spacing, etc.) are set to a single value through the whole zoom range.

Considering that buckets are parsed individually for each tile, which has a zoom value, we can introduce the same functions we use for other rules to bucket rules, with the only difference that the values will be calculated based on tile zoom as opposed to arbitrary fractional zoom. It is also important in case we ditch buckets in user styles, to be consistent.

This issue extends #317

[nickidlugash]

This would be useful for text-offset (and icon-offset) render properties, particularly to transition from symbols with markers + text at low zoom levels to symbols with text only at high zoom levels (like cities with dots) 👍

[lbud]

I assume this would only be for numeric layout properties?

[jfirebaugh]

Yes, for now. Though mapbox/mapbox-gl-style-spec#237 could apply to equally to paint and layout properties.