Blender Git Loki

- Preview render (buttons) was executing the 'data ipos', causing inserting
keys with IKEY in buttons to not work.

- Crash in opengl while rendering was caused by the fact that scanline
updates are drawn in the main thread, whilst the actual render thread
then can already be doing different stuff.
Especially with many layers & passes it's getting confusing easily :)

Convention now is that scanline render updates only happen while the
thread is looping over scanlines. As soon as it reached the last, no
drawing happens, not even to update the last segment.
This isnt a problen, since any finished tile is drawn again entirely.