AGOS: Fix invisibility in Amiga CD32 Simon the Sorcerer 1 demo

This probably affects the full version as well, but I haven't
verified that. Either way, the old code was obviously wrong while
the new makes it look right to me.

engines/agos/gfx.cpp

@@ -547,7 +547,7 @@ void AGOSEngine_Simon1::drawMaskedImage(VC10_state *state) {
 						if ((dst[count * 2] & 0xF0) == 0x20)
 							dst[count * 2] = src[count * 2];
 					if (mask[count + state->x_skip] & 0x0F)
-						if ((dst[count * 2 + 1] & 0x0F) == 0x20)
+						if ((dst[count * 2 + 1] & 0xF0) == 0x20)
 							dst[count * 2 + 1] = src[count * 2 + 1];
 				} else {
 					/* no transparency */