Check if we can always use back.gbcpal

[Rangi42]

data/pokemon/palettes.asm derives its .gbcpal color files from the back.png sprites, since there are separate front_gold.png and front_silver.png ones. But a few back sprites (like Jigglypuff) do not use all four colors, so those are special-cased to just use front_gold.png.

However, I'm not sure if this is actually necessary. When I run rgbgfx -c embedded -p back.pal -o back.2bpp back.png, using rgbgfx 0.6.1, the 2bpp and its palette correctly uses white, light gray, and black, as expected, and the palette even contains the unused fourth color:

$ xxd back.pal
00000000: ff7f 1f7e 067e 0000

Without -c embedded, rgbgfx packs the three used colors into the first three palette slots, sorted by lightness (with the fourth unused slot filled in as ffff):

$ xxd back.pal
00000000: ff7f 1f7e 0000 ffff

To do: see if this is reliable and we can use back.gbcpal with -c embedded even for cases like Jigglypuff. And, does it require 0.6.1 or 0.6.0?

 %.gbcpal: %.png
-	$(RGBGFX) -p $@ $<
+	$(RGBGFX) -c embedded -p $@ $<