Blender Git Loki

Handle grid name lookups from cycles nodes as weak references and
prevent exceptions.

The shader output names are not guaranteed to match the VDB grid names.
The grid info in nodes can get outdated, files can be manipulated
externally etc., so a failure of grid lookup from Cycles should be
handled gracefully.

Fix compile error.

Merge branch 'master' into openvdb

Cycles: use vdb ray intersectors for shadow rays as well.

Fix for crash on NULL pointer if using the high density grid for
rendering.

Store grids' informations in the node.

Patch by @lukastoenne, with minor edits.

Exporter: decrease tolerance yet again.

Cycles: disable -Wfloat-conversion and -Wdouble-promotion for OpenVDB
for now, and quiet warning.

Cycles: de-duplicate ray march loops.

The main logic was split up in a separate function which has quite a
number of arguments, could be worth looking into that matter at some
point. Also it seems like dense volume are slower to render now.

Somewhat improve behaviour of the vdb file reader:

- it now stores a pointer to an io::File to avoid deleting and
reallocating the reader itself for each frame when importing.
- if a file is inder 10 Mb a private copy of it should be made by VDB to
ensure it is not modified while reading. This could become a user
setting at some point.

Correct UI names and tooltips.

Add a dedicated poll function for the cache operators.

Cleanups:

- automatically add an underscore to the filename before the frame
number in the filename instead.
- rename "string" -> "r_filename" argument
- removed useless NULL check

Ask user to overwrite files if they already exist.

Only checks for the frame though.

Fix for recent clip change: ensure fluid matrices are computed for the
export.

Ensure cache directory exists.

Clip some low res grids based on the high res density.

Rationale is that in some cases the high resolution density is larger
than the low resolution one, so here we ensure there's enough data for
lookup during rendering for instance.

Decrease tolerance for vector grids due to possible loss of data.

Increase tolerance value used for copying a dense array to a sparse
grid.

This doesn't affect low resolution grids much but makes a real
difference for high res grids, as it only considers values real close to
the actual smoke or fire.

Although it might be a bit high of a value, it gives some nice file
sizes going from 106.6 Mb down to 37.7 Mb (pointcache: 264.3 Mb) in a
simple simulation with a base res of 128 and a high res of 2.

Merge branch 'master' into openvdb