Removed all limitations from render code for maximum threads. The only define for this now is in BLI_threads.h, and currently set to 8. Note that each thread renders an entire tile, and also allocates the buffers for the tiles, so; more threads might work better with smaller tiles. IMPORTANT: node system won't work yet with more than 2 threads! So, don't try material nodes or compositing with over 2 threads. That I'll commit later today. What does work (should work :) is AO and soft shadow now.