Fix MSVC compile failure on AppVeyor CI.

tiny_gltf.h

diff --git a/tiny_gltf.h b/tiny_gltf.h
index 9e1351b..1e4bb3c 100644
--- a/tiny_gltf.h
+++ b/tiny_gltf.h
@@ -1519,7 +1519,7 @@
   bool preserve_image_channels_ = false;  /// Default false(expand channels to
                                           /// RGBA) for backward compatibility.
 
-  size_t max_external_file_size_{(std::numeric_limits<int32_t>::max)()}; // Default 2GB
+  size_t max_external_file_size_{size_t((std::numeric_limits<int32_t>::max)())}; // Default 2GB
 
   // Warning & error messages
   std::string warn_;