mrview: Bugfix in rendering of cubes in connectome tool

[Lestropie]

Problem introduced in 92eee2a as part of #283.

Have not yet tested for whether it has any effect, just spotted the code while refactoring.

[Lestropie]

Member variable num_indices is read here. So prior to this change, there is presumably a buffer overrun happening, but only on the GPU. Neither gdb nor valgrind catch anything, and the visualisation looks correct in both cases.