Fix e.isSourceLoaded to check if specific source is loaded

[avpeery]

e.isSourceLoaded is intended to return true if a source has no outstanding network requests (see docs for isSourceLoaded in the Map Data Event). Previously, e.isSourceLoaded used this.loaded() from style.js to determine its value. this.loaded() in src/style/style.js however checks all sources to see if they are loaded rather than that specific source id. This fix uses the logic of map.isSourceLoaded to check if that specific source id is loaded.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fix e.isSourceLoaded to check if specific source has loaded.</changelog>

[asheemmamoowala]

The fix looks good, but is missing unit tests. There are similar tests in geojson_source.test.js that could be amended to test all the events from a setData call.

I tested against debug/geojson_update.html and found the following sequence of sourcedata events for a GeoJSON source:

1. sourceDataType: metadata: On first being added to the map and after the worker has loaded inline data or fetched and loaded from remote data.
2. sourceDataType: content: on subsequent calls to setData and after the worker has loaded inline data or fetched and loaded from remote data.
3. sourceDataType: undefined for each tile required to complete rendering the current frame.

e.isSourceLoaded is false for every one of these emitted events until the last tile request is completed.

[asheemmamoowala]

Approved pending an additional unit test with GeoJsonSource#setData showing that isSourceLoaded changes to true.

[avpeery]

@asheemmamoowala Thanks for the review! I added in a call tosetData in the previous test case. If the source is not added to the map before calling setData, isSourceLoaded is not included in the 'data' event response (which is being set here: https://github.com/mapbox/mapbox-gl-js/blob/avpeery/fix-e.isSourceLoaded/src/source/geojson_source.js#L323). While isSourceLoaded is set here: https://github.com/mapbox/mapbox-gl-js/blob/avpeery/fix-e.isSourceLoaded/src/style/style.js#L708. Let me know if this makes sense and if this needs further conversation.