This currently uses the sass-sys low-level interface, because the
operation is not supported in sass-rs.
The PR https://github.com/compass-rs/sass-rs/pull/3 adds high-level
support. If / when that is fixed, the direct requirement for sass-sys
should be removed from rphotos.
The css file is built to a file with (short) checksum in the
file name, to be nicely cacheable. The actual file name is
compiled into the binary and sent to the templates as a variable.
