Blender Git Loki

Cocoa:
- fix 10.6 API used in window resizing callback causing crash on 10.5 systems (Thx Jasper Mine for the bug report)
- implemented min window size enforcement to prevent tiny windows messing up blender's internal ui layout (same as done by Campbell on X11, is a partial fix of bug #19550)
- added (commented) code for enabling multithreaded opengl (this optimization is here for experimental tests, not for mainstream, so bleeding edge testers would want to uncomment the three "Multithreaded opengl code : uncomment for enabling" sections)