Fall back to spec default if property function's target property is undefined

[lbud]

This is an important feature for e.g. buildings styled with

"fill-extrude-height": {
    "stops": [[0, 0], [1000, 1000]]
    "property": "height"
}

Without the ability to fall back to the spec default, features with no height property would extrude to 1000.

(It will also be very important that we land on a solution for mapbox/mapbox-gl-style-spec#480 but, unlike this, not a release blocker IMO.)

[jfirebaugh]

Do we even need to thread the spec default through here, or could evaluating a property function on an undefined property value return undefined, and then upstream code can fill in the spec default?

[lbud]

@jfirebaugh good question — in thinking about this I realized I can work around this for my current needs as described in mapbox/mapbox-gl-style-spec#480 (comment), so I'd be okay with punting on this either way…

[lucaswoj]

@lbud Are you planning to work on this PR further or should it be closed?

[mourner]

Closing as stale for now.