Geometry Nodes: Improve point instance node performance

This commit uses two changes to improve the performance of the point
instance node.

**Prevent Reallocations**
At 64 bytes, the transform matrix for every instance is rather large,
so reallocating the vector as it grows can become a performance bottle-
neck. This commit reserves memory for the instances that will be added
to prevent unecessary reallocations as the instance vector grows.

In a test with 4 million instances of 3 objects in a collection, the
node was about 40% faster, from 370ms to 270ms for the node.

**Parallelization**
Currently the instances are added by appending to a vector. By changing
this slightly to fill indices instead, we can parallelize the operation
so that multiple threads can fill data at the same time. Tested on a
Ryzen 3700x, this reduced the runtime from the above 270ms to 44ms
average, bringing the total speedup to ~8x.

Note that displaying the instances in the viewport is still much slower
than the calculations in node, this change doesn't affect that.

1 files changed, 2 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_geometry_set.hh b/source/blender/blenkernel/BKE_geometry_set.hh
index 106af8172d1..bd2b062152d 100644
--- a/source/blender/blenkernel/BKE_geometry_set.hh
+++ b/source/blender/blenkernel/BKE_geometry_set.hh
@@ -597,6 +597,7 @@ class InstancesComponent : public GeometryComponent {
   void clear();
 
   void reserve(int min_capacity);
+  void resize(int capacity);
 
   int add_reference(InstanceReference reference);
   void add_instance(int instance_handle, const blender::float4x4 &transform, const int id = -1);
@@ -604,6 +605,7 @@ class InstancesComponent : public GeometryComponent {
   blender::Span<InstanceReference> references() const;
 
   blender::Span<int> instance_reference_handles() const;
+  blender::MutableSpan<int> instance_reference_handles();
   blender::MutableSpan<blender::float4x4> instance_transforms();
   blender::Span<blender::float4x4> instance_transforms() const;
   blender::MutableSpan<int> instance_ids();