avfilter/vf_waveform: add acolor filter

Useful in combination with color filter.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -13176,6 +13176,9 @@ Similar as above, but shows difference between blue and red chroma.
 
 @item color
 Displays actual color value on waveform.
+
+@item acolor
+Similar as above, but with luma showing frequency of chroma values.
 @end table
 
 @item graticule, g