sub: add --sub-gray option to display image subs in grayscale

MPlayer/mplayer2 still show DVD subtitles in gray. Depending on who you
ask, this can be considered a bug or a feature. Include rendering in
gray as explicit feature, so the user can decide what is better.

This affects all indexed sub bitmaps entering the OSD rendering path.
Currently, this means all image subs are affected by this option, but
nothing else.

DOCS/man/en/changes.rst

@@ -64,7 +64,7 @@ General changes for mplayer2 to mpv
   handle these use cases. For yuv4mpeg, for example, use:
   ``mpv input.mkv -o output.y4m --no-audio``.
 * Image subtitles (DVDs etc.) are rendered in color and use more correct
-  positioning
+  positioning (color can be disabled with ``--sub-gray``)
 * General code cleanups
 * Many more changes
 

DOCS/man/en/options.rst

@@ -1889,6 +1889,12 @@
 
     *NOTE*: never applied to text subtitles.
 
+--sub-gray
+    Convert image subtitles to grayscale. Can help making yellow DVD/Vobsubs
+    look nicer.
+
+    *NOTE*: never affects text subtitles.
+
 --sub-pos=<0-100>
     Specify the position of subtitles on the screen. The value is the vertical
     position of the subtitle in % of the screen height.