Previously, when the add occurrence modal operator was executed, on
every modal loop (i.e. every mouse movement) it would fetch the mesh
geometry to be previewed, store the verts / edges / faces in mesh
collections, then the decorator would fetch that geometry, the clear the
collections, in a loop.

I've removed the Blender collections. Instead the same strategy is used
as in ItemDecorator i.e. the mesh and verts are fetched once during
decorator installation, then on each draw call only a single vertex loop
to multiply by the transformation matrix for snapping and mouse
position.

You can test with the LOD400 model in #7566. On my machine it would
cause lag on anything with >500 faces. Now it seems to work without lag
on a 26k polygon mesh.