OpenVDB import to padded sparse grids.

Minor bug-fixes and optimizations.

* Change most instances of division and modulo with TILE_SIZE to bit shifting. Much more efficient since TILE_SIZE should be a
power of 2.
* Added intialization for some Mesh members. Previously had motion blur randomly toggle on and off otherwise.
* Fixed issue where voxel to tile correspondence is different between external VDBs and internal sparse grids. The fix requires
generating a new VDB grid which may be too memory intensive. Better method would be translating grids in place.
* Fixed misc OpenVDB to grid conversion issues.

Merge remote-tracking branch 'origin/blender2.8' into soc-2018-cycles-volumes

Updates to OpenVDB import and volume motion blur.

* Separated option for Volume Motion Blur in object UI. Unlike deformation motion, it is disabled by default.
* Added multiple frame import and preview simplying for imported VDBs.
* Fixed a couple of bugs with VDBs in the viewport.

Initial fix for CUDA, OpenCL sparse grid sampling.

Several minor fixes and variable name changes.

Notable changes:
* P_v moved to correct location in ShaderData struct and shader init.
* Volume motion blur now also checks if the Deformation Blur box in
the Object panel is checked.
* Changed OpenVDB variable names to refer to generic volumes.

Cycles support for importing common grid types.

Most common grid types that can easily convert to float are now
supported. These will immediately be stored as float arrays. It is also
easy to add new types of grids in the future if needed.

Remove Cycles dependency on intern/openvdb.

For this, several functions from intern now have simplified copies
in Cycles, since the dependency is otherwise difficult to maintain.

Create SparseTextureInfo.

Will be used for better organization of info used to convert dense to
sparse coordinates in volumes. Also added back tricubic interpolation
of sparse grids.

Remove most VDB grid as texture storage functions.

Fixed incorrect preview of imported OpenVDB files.

Error was caused by not taking into account minumum bound of the volume.
Also changed dense array to VDB grid converter in Cycles to use the
exporter in intern/openvdb.

Move VDB functions from Cycles to intern/openvdb.

So they can be used on Blender side. Changed variable names and types to
be more consistent between Cycles and intern/openvdb.

Clean up. Change VDB iteration method in Cycles to leaf iterators rather than accessors.

Intial commit for volume motion blur.

Cleanup; fix some bugs causing crashes.

Cleanup and bug fixes.

Simple OpenVDB import GUI.

Initial commit for Cycles OpenVDB import.

Revert "Morton ordering for sparse grids."

This reverts commit a93a9fb09bcfd6211f063255ccb11cfc15e013d1.

Morton ordering for sparse grids.