sparse_disp: reduce memory use
sparse_disp: reduce memory use

Change the level in the parallel code where the sub-image pixel centers get cropped.  Now less of the left image gets read by each worker process, and the memory usage goes down from (potentially) tens of GB to hundreds of MB.

njwilson23/StereoPipeline,njwilson23/StereoPipeline,NeoGeographyToolkit/StereoPipeline,DougFirErickson/StereoPipeline,ScottMcMichael/StereoPipeline,DougFirErickson/StereoPipeline,ScottMcMichael/StereoPipeline,aaronknister/StereoPipeline,NeoGeographyToolkit/StereoPipeline,njwilson23/StereoPipeline,aaronknister/StereoPipeline,oleg-alexandrov/StereoPipeline,oleg-alexandrov/StereoPipeline,njwilson23/StereoPipeline,oleg-alexandrov/StereoPipeline,aaronknister/StereoPipeline,DougFirErickson/StereoPipeline,ScottMcMichael/StereoPipeline,njwilson23/StereoPipeline,aaronknister/StereoPipeline,DougFirErickson/StereoPipeline,NeoGeographyToolkit/StereoPipeline,ScottMcMichael/StereoPipeline,DougFirErickson/StereoPipeline,NeoGeographyToolkit/StereoPipeline,NeoGeographyToolkit/StereoPipeline,DougFirErickson/StereoPipeline,NeoGeographyToolkit/StereoPipeline,oleg-alexandrov/StereoPipeline,ScottMcMichael/StereoPipeline,ScottMcMichael/StereoPipeline,oleg-alexandrov/StereoPipeline,aaronknister/StereoPipeline,oleg-alexandrov/StereoPipeline,njwilson23/StereoPipeline