Add Assimp library in thirdparty

From https://github.com/assimp/assimp repo at d2b45377e4b09a1f43be95e45553afcc06b03f4b

thirdparty/assimp/code/MaterialSystem.h

@@ -0,0 +1,72 @@
+/*
+Open Asset Import Library (assimp)
+----------------------------------------------------------------------
+
+Copyright (c) 2006-2019, assimp team
+
+
+All rights reserved.
+
+Redistribution and use of this software in source and binary forms,
+with or without modification, are permitted provided that the
+following conditions are met:
+
+* Redistributions of source code must retain the above
+  copyright notice, this list of conditions and the
+  following disclaimer.
+
+* Redistributions in binary form must reproduce the above
+  copyright notice, this list of conditions and the
+  following disclaimer in the documentation and/or other
+  materials provided with the distribution.
+
+* Neither the name of the assimp team, nor the names of its
+  contributors may be used to endorse or promote products
+  derived from this software without specific prior
+  written permission of the assimp team.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+----------------------------------------------------------------------
+*/
+
+/** @file MaterialSystem.h
+ *  Now that #MaterialHelper is gone, this file only contains some
+ *  internal material utility functions.
+ */
+#ifndef AI_MATERIALSYSTEM_H_INC
+#define AI_MATERIALSYSTEM_H_INC
+
+#include <stdint.h>
+
+struct aiMaterial;
+
+namespace Assimp    {
+
+// ------------------------------------------------------------------------------
+/** Computes a hash (hopefully unique) from all material properties
+ *  The hash value reflects the current property state, so if you add any
+ *  property and call this method again, the resulting hash value will be
+ *  different. The hash is not persistent across different builds and platforms.
+ *
+ *  @param  includeMatName Set to 'true' to take all properties with
+ *    '?' as initial character in their name into account.
+ *    Currently #AI_MATKEY_NAME is the only example.
+ *  @return 32 Bit jash value for the material
+ */
+uint32_t ComputeMaterialHash(const aiMaterial* mat, bool includeMatName = false);
+
+
+} // ! namespace Assimp
+
+#endif //!! AI_MATERIALSYSTEM_H_INC