CustomGeometrySource should store shared_ptr to shared threadpool

[ivovandongen]

In

mapbox-gl-native/src/mbgl/style/sources/custom_geometry_source.cpp

Line 17 in e95bf4d

loader(std::make_unique<Actor<CustomTileLoader>>(*sharedThreadPool(), options.fetchTileFunction, options.cancelTileFunction)) {

a reference to the shared threadpool is handed to the Actor. This could potentially go wrong if the shared threadpool is destroyed before the source is.

To make it safe, it should probably store a shared pointer to the threadpool like here:

mapbox-gl-native/platform/android/src/style/sources/geojson_source.cpp

Lines 51 to 52 in e95bf4d

	, threadPool(sharedThreadPool())
	, converter(std::make_unique<Actor<FeatureConverter>>(*threadPool)) {