When applying a SLD style containing 'Else'-filter, the Else-class will be marked as 'fallback' in Mapserver to ensure this class is used if and only if no other class is applicable.

This MR enables the fallback property to be configured in the mapfile using:

CLASS
...
FALLBACK TRUE
END # CLASS

This makes it possible to reproduce the SLD behavior using FALLBACK together with RENDERMODE=ALL_MATCHING_CLASSES.

I also updated the writeClass method to write FALLBACK and the rendermode directive to keep the SLD behavior intact when mapscript is used to produce mapfiles that have had SLD applied:

map.applySLD(content)
print(map.convertToString())