Remove advance rounding

Closes #2201 

Removes the Mapnik 2.x compatibility rounding from `advance()` in `glyph_info.hpp` per https://github.com/mapnik/mapnik/pull/2272#issuecomment-46520908 and https://github.com/mapnik/mapnik/pull/2272#issuecomment-46590215.

/cc @springmeyer