stb_image: fix unused parameter

stb_image.h

@@ -578,10 +578,10 @@ FILE *stbi__fopen(char const *filename, char const *mode)
 {
    FILE *f;
 #if defined(_MSC_VER) && _MSC_VER >= 1400
-   if (0 != fopen_s(&f, filename, "rb"))
+   if (0 != fopen_s(&f, filename, mode))
       f=0;
 #else
-   f = fopen(filename, "rb");
+   f = fopen(filename, mode);
 #endif
    return f;
 }