Geometry Nodes: refactor multi-threading in field evaluation

Previously, there was a fixed grain size for all multi-functions. That was
not sufficient because some functions could benefit a lot from smaller
grain sizes.

This refactors adds a new `MultiFunction::call_auto` method which has the
same effect as just calling `MultiFunction::call` but additionally figures
out how to execute the specific multi-function efficiently. It determines
a good grain size and decides whether the mask indices should be shifted
or not.

Most multi-function evaluations benefit from this, but medium sized work
loads (1000 - 50000 elements) benefit from it the most. Especially when
expensive multi-functions (e.g. noise) is involved. This is because for
smaller work loads, threading is rarely used and for larger work loads
threading worked fine before already.

With this patch, multi-functions can specify execution hints, that allow
the caller to execute it most efficiently. These execution hints still
have to be added to more functions.

Some performance measurements of a field evaluation involving noise and
math nodes, ordered by the number of elements being evaluated:
```
1,000,000: 133 ms   -> 120 ms
  100,000:  30 ms   ->  18 ms
   10,000:  20 ms   ->   2.7 ms
    1,000:   4 ms   ->   0.5 ms
      100:   0.5 ms ->   0.4 ms
```

1 files changed, 4 insertions, 4 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_transfer_attribute.cc b/source/blender/nodes/geometry/nodes/node_geo_transfer_attribute.cc
index acd8e6cad25..dc5118a56c7 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_transfer_attribute.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_transfer_attribute.cc
@@ -131,9 +131,9 @@ static void get_closest_in_bvhtree(BVHTreeFromMesh &tree_data,
                                    const MutableSpan<float> r_distances_sq,
                                    const MutableSpan<float3> r_positions)
 {
-  BLI_assert(positions.size() == r_indices.size() || r_indices.is_empty());
-  BLI_assert(positions.size() == r_distances_sq.size() || r_distances_sq.is_empty());
-  BLI_assert(positions.size() == r_positions.size() || r_positions.is_empty());
+  BLI_assert(positions.size() >= r_indices.size());
+  BLI_assert(positions.size() >= r_distances_sq.size());
+  BLI_assert(positions.size() >= r_positions.size());
 
   for (const int i : mask) {
     BVHTreeNearest nearest;
@@ -159,7 +159,7 @@ static void get_closest_pointcloud_points(const PointCloud &pointcloud,
                                           const MutableSpan<int> r_indices,
                                           const MutableSpan<float> r_distances_sq)
 {
-  BLI_assert(positions.size() == r_indices.size());
+  BLI_assert(positions.size() >= r_indices.size());
   BLI_assert(pointcloud.totpoint > 0);
 
   BVHTreeFromPointCloud tree_data;