[style-spec] expose convertFilter API

[springmeyer]

This PR exposes the convertFilter at

mapbox-gl-js/src/style-spec/feature_filter/convert.js

Line 14 in b5c9020

export default function convertFilter(filter: FilterSpecification): mixed {

I need this exposed to be able to auto-upgrade legacy filters in a downstream module that uses require('@mapbox/mapbox-gl-style-spec') (mapbox/vtshaver#30)

The larger context is that I need to combine multiple filters into one array to pass them to gl-native. However gl-native cannot cope with mixing legacy filters (see https://docs.mapbox.com/mapbox-gl-js/style-spec/other/) and new style expressions. Previously it crashed and now it throws (mapbox/mapbox-gl-native#12065). But ideally, through auto-upgrading using this newly exposed function, I can avoid gl-native throwing.

Thanks @samanpwbb and @mapbox/studio for recommending this fix.

[asheemmamoowala]

Can you please add unit tests in /test/style-spec?

[ansis]

Looks good to me! If we merge this today it should make it into the next release.

[asheemmamoowala]

Looks like there are already unit tests for this in test/unit/style-spec/feature_filter.test.js