vf_rotate: fix params >= 4
mp_image: copy palette only if allocated

Normally, we assume that IMGFMT_PAL8 always has a palette allocated
in plane 1. But there may be corner cases in ffmpeg where it doesn't
(namely pseudo-pal stuff).
video: add a new method to configure filters and VOs

The filter chain and the video ouputs have config() functions. They are
strictly limited to transfering the video size and format. Other
parameters (like color levels) have to be transferred separately.

Improve upon this by introducing a separate set of reconfig() functions,
which use mp_image_params to carry format parameters. This struct
contains all image format related parameters from config(), plus
additional parameters such as colorspace.

Change vf_rotate to use it, as well as vo_opengl. vf_rotate is just
an example/test case, but vo_opengl will need it later.

The intention is also to get rid of VOCTRL_SET_YUV_COLORSPACE. This
information is now handed to the VOs via reconfig(). The getter,
VOCTRL_GET_YUV_COLORSPACE, will still be needed though.
vo_opengl: handle chroma location

Use the video decoder chroma location flags and render chroma locations
other than centered. Until now, we've always used the intuitive and
obvious centered chroma location, but H.264 uses something else.

FFmpeg provides a small overview in libavcodec/avcodec.h:

-----------
/**
 *  X   X      3 4 X      X are luma samples,
 *             1 2        1-6 are possible chroma positions
 *  X   X      5 6 X      0 is undefined/unknown position
 */
enum AVChromaLocation{
    AVCHROMA_LOC_UNSPECIFIED = 0,
    AVCHROMA_LOC_LEFT        = 1, ///< mpeg2/4, h264 default
    AVCHROMA_LOC_CENTER      = 2, ///< mpeg1, jpeg, h263
    AVCHROMA_LOC_TOPLEFT     = 3, ///< DV
    AVCHROMA_LOC_TOP         = 4,
    AVCHROMA_LOC_BOTTOMLEFT  = 5,
    AVCHROMA_LOC_BOTTOM      = 6,
    AVCHROMA_LOC_NB             , ///< Not part of ABI
};
-----------

The visual difference is literally minimal, but since videophiles
apparently consider this detail as quality mark of a video renderer,
support it anyway. We don't bother with chroma locations other than
centered and left, though.

Not sure about correctness, but it's probably ok.
csputils.h: don't recursively include libavcodec header

Some functions (avcol_spc_to_mp_csp() etc.) used libavcodec enum types
as parameters. Remove these in order to get rid of the avcodec.h
include statement. This prevents that avcodec.h is recursively
included by dozens of files. Fix mp_image.c, which used the header
without explicitly including avcodec.h.