[core] Assert fires in LayerFactory::createBucket

[julianrex]

As of 40b0cb7, when running the iOS Integration Test Harness tests, specifically testForRaisingExceptionsOnStaleLayerObject, there's an assert in:

mapbox-gl-native/src/mbgl/layermanager/layer_factory.cpp

Lines 28 to 31 in 40b0cb7

	std::unique_ptr<Bucket> LayerFactory::createBucket(const BucketParameters&, const std::vector<Immutable<style::LayerProperties>>&) noexcept {
	assert(false);
	return nullptr;
	}

for a factory of type RasterStyleLayerPeerFactory. Missing function?

@tmpsantos suggested this may be a regression introduced in #15308

/cc @tobrun

[pozdnyakov]

The reason is that we allow adding layers incompatible with sources. In the test a raster layer is added to a vector source.