openvdb_view : add support for zooming without the scroll wheel?

i am not so used to using the scroll wheel for zooming, i dont mind using the scroll wheel for shifting the cut planes but not for navigation. maybe add support both?

Start by locating the openvdb_view navigation and input-handling entry points, then inspect how the scroll wheel currently controls zooming and cut-plane shifting. Determine which alternate zoom interaction should be supported and whether both controls can coexist; done means navigation zoom works without requiring the scroll wheel while cut-plane behavior remains usable.

Written by the indexing model from the issue text.