better support for ogr layer types for which millstone cannot safely assume a single layer - closes #1754
better support for ogr layer types for which millstone cannot safely assume a single layer - closes #1754

Teino1978-Corp/Teino1978-Corp-tilemill,fxtentacle/tilemill,MappingKat/tilemill,mbrukman/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,fxtentacle/tilemill,mbrukman/tilemill,tizzybec/tilemill,paulovieira/tilemill-clima,florianf/tileoven,mbrukman/tilemill,tizzybec/tilemill,paulovieira/tilemill-clima,MappingKat/tilemill,MappingKat/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,Zhao-Qi/tilemill,MappingKat/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,fxtentacle/tilemill,nyimbi/tilemill,fxtentacle/tilemill,Zhao-Qi/tilemill,tizzybec/tilemill,nyimbi/tilemill,florianf/tileoven,mbrukman/tilemill,mbrukman/tilemill,fxtentacle/tilemill,Zhao-Qi/tilemill,nyimbi/tilemill,florianf/tileoven,tizzybec/tilemill,paulovieira/tilemill-clima,tilemill-project/tilemill,tilemill-project/tilemill,nyimbi/tilemill,paulovieira/tilemill-clima,Zhao-Qi/tilemill,Teino1978-Corp/Teino1978-Corp-tilemill,MappingKat/tilemill,nyimbi/tilemill,tizzybec/tilemill,Zhao-Qi/tilemill,tilemill-project/tilemill,paulovieira/tilemill-clima