Fix signed/unsigned issue on non-INTC processors

Some ugliness in different GCC versions caused the interpolation steps
to be calculated incorrectly.  Explicitly move through variable of the
right type to ensure proper signedness of operations.

src/render.c

@@ -678,9 +678,11 @@ do
 			// ML : Code47503 is division with remainder, and mem50 gets the sign
 
 			// calculate change per frame
+			signed char m53 = (signed char)mem53;
 			mem50 = mem53 & 128;
-			mem51 = abs((char)mem53) % mem40;
-			mem53 = (unsigned char)((char)(mem53) / mem40);
+			unsigned char m53abs = abs(m53);
+			mem51 = m53abs % mem40; //abs((char)m53) % mem40;
+			mem53 = (unsigned char)((signed char)(m53) / mem40);
 
             // interpolation range
 			X = mem40; // number of frames to interpolate over