vo.c, vo.h, vo_null.c: change license to LGPL

Most contributors have agreed. vo.c requires a "driver" entry for each
video output - we assume that if someone who didn't agree to LGPL added
a line, it's fine for vo.c to be LGPL anyway. If the affected video
output is not disabled at compilation time, the resulting binary will be
GPL anyway.

One problem are the changes by Nick Kurshev (usually using "nick" as SVN
username). He could not be reached. I believe all changes to his files
are actually gone, but here is a detailed listing:

fa1d5742bc: nick introduces a new VO API. It was removed in 64bedd9683.
Some of this was replaced by VOCTRLs are introduced in 7c51652a1b,
obviously replacing at least some functionality by his API.

b587a3d642: nick adds a vo_tune_info_t struct. Removed in 64bedd9683
too.

9caad2c29a: nick adds some VOCTRLs, which were silently removed in
8cc5ba5ab8 (they became unused probably with the VIDIX removal).

340183b0e9: nick adds VO-based screenshots, which got removed in
2f4b840f62. Strangely the same name was introduced in 01cf896a2f again,
but this is a coincidence and worked differently (also it was removed
yet again in 2858232220).

104c125e6d: nick adds an option for "direct rendering". It was renamed
in 6403904ae9 and fully removed in e48b21dd87.

5ddd8e92a1: nick adds code to check the VO driver preinit arg to every
single VO driver. The argument itself and any possibly remaining code
associated with it was removed in 1f5ffe7d30.

f6878753fb: nick adds header inclusion guards. We assume this is not
relevant for copyright.

Some of nick's code was merely moved to other files, such as the
equalizer stuff added in 555c676683 and moved in 4db72f6a80 and
12579136ff, and don't affect copyright of these files anymore.

Other notes:

fef7b17c34: a patch by someone who wasn't asked for relicensing added a
symbol that was removed again in 1b09f4633.

4a8a46fafd: author probably didn't agree to LGPL, but the function
signature was changed later on anyway, and nothing of this is left.

7b25afd742: the same author adds a symbol to what is vo.h today, which
this relicensing commit removes, as it was unused. (It's not clear
whether the mere symbol is copyrightable, but no need to take a risk.)

3a406e94d7, 9dd8f241ac: slave mode things by someone who couldn't be
reached. This aspect of the old slave mode was completely removed.

bbeb54d80a: patch by someone who was not asked, but the added code was
completely removed again.

Copyright

@@ -301,14 +301,14 @@ LGPL relicensing status:
     video/out/drm_common.*          LGPL
     video/out/filter_kernels.*      LGPL (BSD)
     video/out/opengl/*              LGPL
-    video/out/vo.c                  hard
-    video/out/vo.h                  hard
+    video/out/vo.c                  LGPL
+    video/out/vo.h                  LGPL
     video/out/vo_caca.c             unknown
     video/out/vo_direct3d.c         unknown
     video/out/vo_drm.c              LGPL
     video/out/vo_image.c            unknown
     video/out/vo_lavc.c             unknown
-    video/out/vo_null.c             unknown
+    video/out/vo_null.c             LGPL
     video/out/vo_opengl.c           LGPL
     video/out/vo_opengl_cb.c        LGPL
     video/out/vo_rpi.c              LGPL