Stereo rendering functionality.

Use '--stereo" option to render horizontal or vertical pair. Also posible to specify left or right eye to render.

Note: Unfortunately, Puli's taskgroup functionality is broken, so we have to construct a plain graph with non-fancy dependencies. We can get back to taskgroups if they will be fixed.
Note2: This commit also implements profile switching functionality. It isn't completely done yet. Will be finished later.