Blender Git Loki

Couple of features/fixes:

- Compositor now frees memory of buffers internally used in groups
immediately. This wasn't part of the event-based cache anyway

- New option: "Free Texture Images" (in render Output panel). This
frees after each render of each scene all images and mipmaps as
used by textures. As reference it prints total amount of MB freed.

- Render stage 'creating speed vectors' had no ESC checking yet

- Made drawing scanline updates during render draw 1 scanline less...
dunno, still hunting for weird opengl crashes.

- 3D preview render didn't properly skip sequence or composit render.