librender: Don't clear z buffer if it is not allocated
librender: Don't clear z buffer if it is not allocated

Also cleaned up RenderContext::fillTile a bit

FulcronZ/NyuziProcessor,jbush001/NyuziProcessor,FulcronZ/NyuziProcessor,FulcronZ/NyuziProcessor,jbush001/NyuziProcessor,hoangt/NyuziProcessor,jbush001/NyuziProcessor,hoangt/NyuziProcessor,FulcronZ/NyuziProcessor,hoangt/NyuziProcessor,jbush001/NyuziProcessor,hoangt/NyuziProcessor,jbush001/NyuziProcessor,jbush001/NyuziProcessor