Blender Git Loki

UI: Changes to Viewport Display UI of Volume Object and Fluid

- Density, interpolation and slicing options in Volume Object's
Viewport Display are now not aligned to each other as they are not
closely related.
- Changed the enum property for slicing ('Method') to a boolean
property 'Slice' and added a sub-panel for slicing options under
the 'Viewport Display' panel in Volume Object and Fluid for better
clarity.
- Renamed `axis_slice_method` to `use_slice` in the python API for
Volume Object and Fluid.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D9097