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.

1 files changed, 4 insertions, 0 deletions

diff --git a/DOCS/man/en/vo.rst b/DOCS/man/en/vo.rst
index 079e146f8a..3b2c280724 100644
--- a/DOCS/man/en/vo.rst
+++ b/DOCS/man/en/vo.rst
@@ -465,6 +465,10 @@ opengl
         that when using FBO indirections (such as with ``opengl-hq``), a FBO
         format with alpha must be specified with the ``fbo-format`` option.
 
+    chroma-location=<auto|center|left>
+        Set the YUV chroma sample location. auto means use the bitstream
+        flags (default: auto).
+
 opengl-hq
     Same as ``opengl``, but with default settings for high quality rendering.