Make histogram thread a little safer from the main thread deleting data
Make histogram thread a little safer from the main thread deleting data

At the start of the background thread's create histogram code, increment
the reference count on the data array we are creating a histogram of so
that it will not be deleted by the main thread while we are working on
it.

thewtex/tomviz,cjh1/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,cryos/tomviz,cryos/tomviz,mathturtle/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,cryos/tomviz,cjh1/tomviz,thewtex/tomviz,cjh1/tomviz,thewtex/tomviz