bugfix, serializer skipped texture samplers refering to first index.

tiny_gltf.h

diff --git a/tiny_gltf.h b/tiny_gltf.h
index 25f9bf4..2af9664 100644
--- a/tiny_gltf.h
+++ b/tiny_gltf.h
@@ -3910,7 +3910,7 @@
 }
 
 static void SerializeGltfTexture(Texture &texture, json &o) {
-  if (texture.sampler > 0) {
+  if (texture.sampler > -1) {
     SerializeNumberProperty("sampler", texture.sampler, o);
   }
   SerializeNumberProperty("source", texture.source, o);