port simplified text-font static output validation

[chloekraw]

Per mapbox/mapbox-gl-js#9355, possibleOutputs is used in GL-Native in at least one place where tracking possibleOutputs as an array may be an overkill:

mapbox-gl-native/src/mbgl/style/layers/symbol_layer_impl.cpp

Lines 36 to 40 in 8be1352

	[&] (const auto& function) {
	for (const auto& value : function.possibleOutputs()) {
	if (value) {
	fontStack.insert(*value);
	} else {

[stale]

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