T39690: Modifications to Blender's 'temp dir' system.

Current temporary data of Blender suffers one major issue - default 'temp' dir on Windows is never
automatically cleaned up, and can end being quite big when used by Blender, especially when we have
to store per-process data (using getpid() in file names).

To address this, this patch:
* Divides tempdir paths in two, one for 'base' temp dir (the same as previous unique tempdir path),
  the other is a mkdtemp-generated sub-dir, specific to each Blender instance.
* Only uses base tempdir when we need some shallow persistance accross Blender sessions - and we always
  reuse the same filename (quit.blend...) or generate small file (crash reports...).
* Uses temp sub-dir for heavy files like pointcache or renderEXRs (Save Buffer option).
* Erases temp sub-dir on quit or crash.

To get this working it also adds a working 'recursive delete' to BLI_delete() under Windows.

Note that, as in current code, the 'recover render result' hack-feature that was possible
with SaveBuffer option is still removed. A real renderresult cache feature will be added
soon, though.

Reviewers: campbellbarton, brecht, sergey

Reviewed By: campbellbarton, sergey

CC: sergey

Differential Revision: https://developer.blender.org/D531

source/blender/python/intern/bpy_app.c

@@ -250,7 +250,7 @@ PyDoc_STRVAR(bpy_app_tempdir_doc,
 );
 static PyObject *bpy_app_tempdir_get(PyObject *UNUSED(self), void *UNUSED(closure))
 {
-	return PyC_UnicodeFromByte(BLI_temporary_dir());
+	return PyC_UnicodeFromByte(BLI_temp_dir_session());
 }
 
 PyDoc_STRVAR(bpy_app_driver_dict_doc,