BLI_kdtree: refactor to support different numbers of dimensions

This moves logic into kdtree_impl.h which is included in a source
file that defines the number of dimensions - so we can easily support
different numbers of dimensions as needed
(currently 3D and 4D are supported).

Macro use isn't so nice but avoids a lot of duplicate code.

source/blender/blenlib/intern/kdtree_3d.c

@@ -0,0 +1,25 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+/** \file
+ * \ingroup bli
+ */
+
+#define KD_DIMS 3
+#define KDTREE_PREFIX_ID BLI_kdtree
+#  include "kdtree_impl.h"
+#undef DIMS
+#undef KDTREE_PREFIX_ID