Code was trying to convert 257.023810 to 'unsigned char'
during vtkRenderingVolumePython-TestBunykRayCastFunction test.

An off-by-one error was causing each loop to increment
a bit too much, and thus not ending at 255.0.

Found by clang ubsan; fix by David Gobbi.

Change-Id: I9af8ee13ba687e2d2ca94705214cd56549e1e44f