merge and update all copies of stb_image.h

code/AssetLib/M3D/M3DWrapper.h

@@ -56,6 +56,10 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 //#define ASSIMP_USE_M3D_READFILECB
 //#define M3D_ASCII
 
+// Share stb_image's PNG loader with other importers/exporters instead of bringing our own copy.
+#define STBI_ONLY_PNG
+#include <contrib/stb_image/stb_image.h>
+
 #include "m3d.h"
 
 namespace Assimp {