Fix compilation when MNG support is active.

src/include/unittype.h

@@ -930,8 +930,8 @@ public:
 		std::string *Files;
 		int Num;
 		Mng **Mngs;
-		int CurrMng;
-		int NumIterations;
+		mutable int CurrMng;
+		mutable int NumIterations;
 	} Portrait;
 #endif
 	MissileConfig Missile;                           /// Missile weapon

src/video/mng.cpp

@@ -316,7 +316,6 @@ void Mng::Draw(int x, int y)
 */
 int Mng::Load(const std::string &name)
 {
-	mng_retcode myretcode;
 	char buf[PATH_MAX];
 
 	LibraryFileName(name.c_str(), buf, sizeof(buf));
@@ -339,7 +338,7 @@ int Mng::Load(const std::string &name)
 
 	mng_read(handle);
 	if (surface && iteration != 0x7fffffff) {
-		myretcode = mng_display(handle);
+		mng_display(handle);
 	}
 
 	if (!surface || iteration == 0x7fffffff) {