RangeTree API rewrite

Rewrite the current range-tree API used by dyn-topo undo
to avoid inefficiencies from stdc++'s set use.

- every call to `take_any` (called for all verts & faces)
  removed and added to the set.
- further range adjustment also took 2x btree edits.

This patch inlines a btree which is modified in-place,
so common resizing operations don't need to perform a remove & insert.
Ranges are stored in a list so `take_any` can access the first item
without a btree lookup.

Since range-tree isn't a bottleneck in sculpting, this only gives minor speedups.
Measured approx ~15% overall faster calculation for sculpting,
although this number time doesn't include GPU updates and depends on how
much edits fragment the range-tree.

1 files changed, 3 insertions, 3 deletions

diff --git a/extern/rangetree/CMakeLists.txt b/extern/rangetree/CMakeLists.txt
index ba682233381..e8e9b15f8b1 100644
--- a/extern/rangetree/CMakeLists.txt
+++ b/extern/rangetree/CMakeLists.txt
@@ -21,10 +21,10 @@ set(INC
 )
 
 set(SRC
-	range_tree.hh
-	range_tree_c_api.h
+	range_tree.h
+	intern/generic_alloc_impl.h
 
-	range_tree_c_api.cc
+	intern/range_tree.c
 )
 
 blender_add_lib(extern_rangetree "${SRC}" "${INC}" "")