OpenVDB: Fixed in-memory rendering.

OpenVDB: Applied `make format` to apply the correct code style.

OpenVDB: Enabled viewport drawing.

OpenVDB: Fixed Windows build.

OpenVDB modifier: Fixed path resolution

Making OpenVDB compile.

Cycles: VDB lookup transformation is now entirely derived from Blender input, no more lookups into VDB world transform. This fixes some VDB files that rendered incorrectly.

Cycles: fixed Linux build with gcc

Cycles: Fixed VDB grid lookup when several grids in a file have the same name

Cycles: Changed isovalue compaison for volume meshes to include equals - probably suboptimal volume meshes, but correct renders at least

OpenVDB modifier: Changed the "sparse render" name to "in memory render", defaulting to sparse data structure

Cycles: Enabled direct sampling of OpenVDB data in the CPU kernel.

OpenVDB: Disable simplification when rendering sparse

Simplification would mess up some values that Cycles needs when doing
sparse renders, so this commit disables render simplification when
sparse rendering is enabled, ensuring the values will be correct.

OpenVDB: Expose `index_offset` property in RNA

OpenVDB: Set `clipping` to 0.0

OpenVDB: Make filepath of current frame available

This makes `abs_path` in the OpenVDB modifier return the path to the
frame that should currently be read (taking into account the current
frame, the frame offset, and frame overrides).

This is a bit hackish, as it duplicates the frame calculation and path
generation code, but it was the quickest and easiest way to achieve
this, in contrast to adapting and exposing the existing functions to
work with this use-case.

OpenVDB: Add sparse render option

This option prevents the cache from being loaded into memory.

OpenVDB: Make "simplify" available for render

Cycles: Fixed OpenVDB path names, there was a possible memory corruption when generating the absolute path to a VDB file.

Cycles: More work on OpenVDB rendering.

Moved it out of ImageManager to a VolumeManager. Added custom transforms to OpenVDB to swap and mirror XYZ.