GUI: Change the undithering checkbox to be an engine-specific option

engines/sci/detection.cpp

@@ -364,6 +364,16 @@ Common::String convertSierraGameId(Common::String sierraId, uint32 *gameFlags, R
 #include "sci/detection_tables.h"
 
 static const ADExtraGuiOptionsMap optionsList[] = {
+	{
+		GAMEOPTION_EGA_UNDITHER,
+		{
+			_s("EGA undithering"),
+			_s("Enable undithering in EGA games"),
+			"disable_dithering",
+			false
+		}
+	},
+
 	{
 		GAMEOPTION_PREFER_DIGITAL_SFX,
 		{