Some RGB conversion are broken if SWS_ACCURATE_RND is not set.
This commit should be reverted when swscale is fixed (or at least a
check on the version is needed).

This will not change anything, since this is the same codec in FFmpeg, but seeing that VP30 is way more used than VP31, the codec display will be more oftenly accurate.

This apply to teletext and DVD menus buttons.
Close #2516.

This is consistent with the vlcrc syntax. Shell scripts et al. are
unlikely to be aware of the locale decimal separator anyway.

This avoids the ugly blinking while resizing.
The embedding window should set the proper background in any case.

It will avoid some useless copy in video_output.

It seems that the field AVFrame::reference may change between the 2 calls.

AC_MSG_FAILURE is solely meant for configure check unexpected failure,
not for "normal" errors whereby a check detects a missing dependency.

It made things more complicated for MIT-SHM.

(Those should not have been in POTFILES in the first place)