Assimp master head fixes for failure to compile (#5899)

* Fix unknown pragma error on msys2 mingw

* Fix missing assignment operator on msys2 mingw

Default is omitted by compiler because custom copy constructor

* Fix redefinition of _1 and _2 error on msys2 mingw

fix is converting it to lambda as clang is claiming that is
preferred

* Fix strncpy warnings about truncation

* Fix missing assignment operator error on clang

* Update glTFImporter.cpp

* Update glTF2Importer.cpp

---------

Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>

contrib/clipper/clipper.cpp

@@ -264,7 +264,14 @@ class Int128
     Int128(const Int128 &val): lo(val.lo), hi(val.hi){}
 
     Int128(const long64& _hi, const ulong64& _lo): lo(_lo), hi(_hi){}
-    
+
+  Int128& operator = (const Int128 &val)
+    {
+      lo = val.lo;
+      hi = val.hi;
+      return *this;
+    }
+
     Int128& operator = (const long64 &val)
     {
       lo = (ulong64)val;