ImageAlgo : Fix tile range for `parallelProcessTiles`
ImageAlgo : Fix tile range for `parallelProcessTiles`

This overload was unused until I tried to use it today. The overload below _is_ used in ImagePlug and ImageGadget and has the correct calculation for `numTiles`, which I've taken as the fix.

andrewkaufman/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,boberfly/gaffer,appleseedhq/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,hradec/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,ImageEngine/gaffer,boberfly/gaffer,ImageEngine/gaffer,hradec/gaffer,johnhaddon/gaffer,boberfly/gaffer,lucienfostier/gaffer,hradec/gaffer,hradec/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,boberfly/gaffer,hradec/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer