Decrement operator for KisTiledHLineIterator

[Bart Coppens]

As I was looking if changing the algorithm of the flood fill to something 
stackbased could make it faster, I noticed the operator-- of the 
KisHLineIterator was not implemented.
Attached is a patch, but I'm getting the impression that I made an error 
somewhere, because I'm getting very weird results with this code. Maybe 
someone could have a look at it?
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