Add clamping to rasterization to account for bad input boxes
Add clamping to rasterization to account for bad input boxes

After deleting a data source some of its data (i.e. chart bounds) are
retained by the 2D transfer function chart.  Add clamping to the bounds
used in rasterization to avoid these leftovers causing segfaults when
data is loaded later (the invalid bounds leads to an invalid default box
and invalid indices being computed and set accessing unallocated
memory).

OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz