spiv: Print backend help if only '-b help' was passed

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
demos: Fix and cleanup GP_BackendInit() + docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Add RGB tripplet to pixel conversion docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>