Implement grayscale DMG palette specs (#1709)

man/rgbgfx.1

@@ -159,6 +159,24 @@ is the case-insensitive word
 then the first four colors of the input PNG's embedded palette are used.
 It is an error if the PNG is not indexed, or if colors other than these 4 are used.
 .Pq This is different from the default behavior of indexed PNGs, as then unused entries in the embedded palette are ignored, whereas they are not with Fl c Cm embedded .
+.It Sy DMG palette spec
+If
+.Ar pal_spec
+starts with case-insensitive
+.Cm dmg= ,
+then the following two-digit hexadecimal number specifies four grayscale DMG color indexes.
+The number functions like the DMG's $FF47
+.Sy BGP
+register
+(see
+.Lk https://gbdev.io/pandocs/Palettes.html Pan Docs
+for more information):
+the low two bits 0-1 specify which gray shade goes in color index 0,
+the next two bits 2-3 specify which gray shade goes in color index 1,
+and so on.
+Gray shade 0 is the lightest (white), 3 is the darkest (black).
+The same gray shade cannot go in two color indexes.
+To specify a DMG palette, the input PNG must have all its colors in shades of gray, without any transparent colors.
 .It Sy external palette spec
 Otherwise,
 .Ar pal_spec
@@ -528,6 +546,8 @@ Otherwise, if the PNG only contains shades of gray, they will be categorized int
 .Dq bins
 as there are colors per palette, and the palette is set to these bins.
 The darkest gray will end up in bin #0, and so on; note that this is the opposite of the RGB method below.
+This is equivalent to having specified a DMG palette of
+.Fl c Cm dmg=E4 .
 If two distinct grays end up in the same bin, the RGB method is used instead.
 .Pp
 Be careful that