StdPNG: Fix crash when saving a screenshot on Linux/Mac 64bit

"unsigned long" is 8 bytes on 64-bit Linux and Mac, so we write 4 bytes
past the end of the allocated memory region.

src/graphics/StdPNG.cpp

@@ -225,7 +225,7 @@ bool CPNGFile::SetPix(int iX, int iY, DWORD dwValue)
 		pPix[2] = GetRedValue(dwValue);
 		return true;
 	case PNG_COLOR_TYPE_RGB_ALPHA: // RGBA: simply set in mem
-		*(unsigned long *) pPix = dwValue;
+		*(DWORD *) pPix = dwValue;
 		return true;
 	}
 	return false;