Add a new "Swap compression" option to the preferences, allowing
explicit control over the tile-swap compression algorithm.
Previously, control over swap compression was only possible through
GEGL command-line options/environment variables.  Since the GEGL
API to list all available compression algorithms is still private
for now, we currently only list the three predefined compression
levels -- "best performance" (the default), "balanced", and "best
compression" -- and a "none" option, to disable compression
altogether.  Selecting a custom compression algorithm is possible
by entering its name manually.