fix(sgi): Fix valid_file to properly swap bytes on little-endian platforms (#4697)

The SGI IRIS format is big-endian so incoming multi-byte fields need to
be swapped. This wasn't being done in the `valid_file` call so the magic
test would always fail on little-endian platforms.

Signed-off-by: Jesse Yurkovich <jesse.y@gmail.com>

Author: jessey-git

src/sgi.imageio/sgiinput.cpp

@@ -89,6 +89,9 @@ SgiInput::valid_file(Filesystem::IOProxy* ioproxy) const
 
     int16_t magic {};
     const size_t numRead = ioproxy->pread(&magic, sizeof(magic), 0);
+    if (littleendian()) {
+        swap_endian(&magic);
+    }
     return numRead == sizeof(magic) && magic == sgi_pvt::SGI_MAGIC;
 }