Add option to disable histogram regneration

[sgpearse]

To reproduce:

1. On Casper, open the session file in /glade/campaign/cisl/vast/vapor/Bugs/3612
2. Increase the region size. The larger the region, the longer VAPOR will take to render because it's sampling more data (presumably for populating the histogram)

Very large datasets can take several minutes for each change to a renderer's region, due to VAPOR recalculating the histogram. This happens even when a renderer is disabled. There needs to be a way to disable this such as simply not doing it when a renderer is turned off, or having a checkbox in the transfer function to not automatically recalculate.