Fix inconsistent handling of transparent colors in the middle of palettes

include/gfx/pal_spec.hpp

@@ -4,8 +4,12 @@
 #define RGBDS_GFX_PAL_SPEC_HPP
 
 #include <stdint.h>
+#include <string>
+
+#include "gfx/png.hpp"
 
 void parseInlinePalSpec(char const * const rawArg);
+void parseEmbeddedPalSpec(Png const &png);
 void parseExternalPalSpec(char const *arg);
 void parseDmgPalSpec(char const * const rawArg);
 void parseBackgroundPalSpec(char const *arg);