topojson sources: support all single-layer sources (don't hardcode single layers to 'vectile')
topojson sources: support all single-layer sources
(don't hardcode single layers to 'vectile')

xvilan/tangram,taylorhxu/tangram,TitoAgudelo/tangram,taylorhxu/tangram,rosamcgee/tangram,TitoAgudelo/tangram,rosamcgee/tangram,taylorhxu/tangram,ceseale/tangram,tangrams/tangram,xvilan/tangram,ceseale/tangram,zerebubuth/tangram,xvilan/tangram,tangrams/tangram,zerebubuth/tangram,ceseale/tangram,TitoAgudelo/tangram,zerebubuth/tangram,rosamcgee/tangram