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/windowmanager/intern/wm_operators.c

@@ -2587,7 +2587,7 @@ void WM_recover_last_session(bContext *C, ReportList *reports)
 {
 	char filepath[FILE_MAX];
 	
-	BLI_make_file_string("/", filepath, BLI_temporary_dir(), BLENDER_QUIT_FILE);
+	BLI_make_file_string("/", filepath, BLI_temp_dir_base(), BLENDER_QUIT_FILE);
 	/* if reports==NULL, it's called directly without operator, we add a quick check here */
 	if (reports || BLI_exists(filepath)) {
 		G.fileflags |= G_FILE_RECOVER;