multiview: Add option to swap left/right stereo channels (UI only)

"Interlace", "Side-By-Side" and "Anaglyph" each have its own "swap left/right" option so user can, for example, quickly switch between cross-eyed "Side-By-Side" and typical anaglyph scheme. For anaglyph the option only available for green-magenta because it does not make sense for other schemes.

Signed-off-by: Alexey Akishin <alex@science.su>