Blender Git Loki

Compositor: finished work on node "File Output".

- It saves a file with indicated type on each change, with number
appended denoting the current frame (like ANIM saving).
- Output filename button supports relative paths ("//")
- Shows optional preview image too
- For now, added a print on each file save as feedback

To make this option work nicely, changed the BKE_makepicstring() function
to have less globals inside, so it is more generic. Todo: allow amount of
digits in filenames to be set (to support files like tmp_123456.jpg)