R_Char2Frame special case for backslash: accept plus as substitute

src/r_things.h

@@ -219,6 +219,7 @@ FUNCMATH FUNCINLINE static ATTRINLINE char R_Frame2Char(UINT8 frame)
 FUNCMATH FUNCINLINE static ATTRINLINE UINT8 R_Char2Frame(char cn)
 {
 #if 1 // 2.1 compat
+	if (cn == '+') return '\\' - 'A'; // PK3 can't use backslash, so use + instead
 	return cn - 'A';
 #else
 	if (cn >= 'A' && cn <= 'Z') return cn - 'A';