error if sf is not loaded


This errors

```r
library(mapdeck)
df <- roads
encode(df[1:200, ])
```

But this works (the internals)

```r
library(mapdeck)
df <- roads
encode(df[1:200, ])
geomCol <- googlePolylines:::sfGeometryColumn(df[1:200, ])
lst <- googlePolylines:::rcpp_encodeSfGeometry(df[[geomCol]], FALSE)
```