Image expression needs to be fully compatible with styleimagemissing event

[ryanhamley]

Motivation

The image operator introduced in #8684 can sometimes short-circuit the styleimagemissing operator, even if no image is found. This happens because the operator returns null when an image is not found in the style.

Design

We should recognize when an image operator evaluates to null and determine which image was requested, then return that image's name, instead of null. This needs to happen after evaluation so that the image operator works with coalesce statements. In such a statement, there will likely be several images requested. A simple heuristic can be used to determine which image we should fire styleimagemissing for; we should be able to treat an expression with several images in it as analogous to a font-stack in which we assume that the most desired option is the first one listed, so we should determine what the first image requested was and use that to fire styleimagemissing.

Concepts

#8684 #7987 and the coalesce operator are the key concepts

Implementation

I have a pretty good jump on implementation. I think the best place to implement this is within the getValueAndResolveTokens method:

mapbox-gl-js/src/style/style_layer/symbol_style_layer.js

Lines 95 to 103 in 5edbaf1

	getValueAndResolveTokens(name: *, feature: Feature, availableImages: Array<string>) {
	const value = this.layout.get(name).evaluate(feature, {}, availableImages);
	const unevaluated = this._unevaluatedLayout._values[name];
	if (!unevaluated.isDataDriven() && !isExpression(unevaluated.value)) {
	return resolveTokens(feature.properties, value);
	}
	return value;
	}

cc @asheemmamoowala @alexshalamov