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. I would rather add a real render caching feature,
but this would go in another patch!