Assimp FBX Import support

Issues fixed:
- Updated assimp to latest and backported fixes into godot.
- Fixed file scale being ignored from FBX file.
- Fixed bone removal
- Implemented proper armature binding
- Fixed recursion not always going through the entire path
- Implemented assimp global scaling system
- Fixed assimp global scale process to support unit conversion
- Implemented proper fbx scaling
- Fixed asserts caused by missing faces in some models which could crash
- Fixed valid bone removal
- Fixed root node being overwriten by assimp which caused data loss
- Fixed armature construction so that it works with multiple roots
- Implemented basic support for FBX standard materials
- Refactoring to improve code quality and improve function reuse.
- Simplified node creation from assimp scene into subsections: create_light, create_mesh, create_bone.
- Creating meshes is now done after hierarchy is created so that the skeleton is always available.
- Added support to assimp to support file scale in all formats which call SetFileScale.
- Many other fixes provided into assimp.

Known issues:
- FBX pivots from Maya do not currently work. (workaround: for now use blender import and export to remove pivot tracks)
- Hierarchy creates an extra node for each mesh - this was done intentionally but we intended to do a pass to remove these as they're a required node.
- When an animated mesh has not executed any animation the rest pose is wrong.

Co-authored-by: K. S. Ernest (iFire) Lee <ernest.lee@chibifire.com>

thirdparty/assimp/code/PostProcessing/ScaleProcess.h

@@ -39,7 +39,8 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 ----------------------------------------------------------------------
 */
-#pragma once
+#ifndef SCALE_PROCESS_H_
+#define SCALE_PROCESS_H_
 
 #include "Common/BaseProcess.h"
 
@@ -53,6 +54,11 @@ namespace Assimp {
 
 // ---------------------------------------------------------------------------
 /** ScaleProcess: Class to rescale the whole model.
+ * Now rescales animations, bones, and blend shapes properly.
+ * Please note this will not write to 'scale' transform it will rewrite mesh 
+ * and matrixes so that your scale values 
+ * from your model package are preserved, so this is completely intentional
+ * bugs should be reported as soon as they are found.
 */
 class ASSIMP_API ScaleProcess : public BaseProcess {
 public:
@@ -78,7 +84,7 @@ public:
     virtual void Execute( aiScene* pScene );
 
 private:
-    void traverseNodes( aiNode *currentNode );
+    void traverseNodes( aiNode *currentNode, unsigned int nested_node_id = 0 );
     void applyScaling( aiNode *currentNode );
 
 private:
@@ -86,3 +92,6 @@ private:
 };
 
 } // Namespace Assimp
+
+
+#endif // SCALE_PROCESS_H_