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/contrib/utf8cpp/doc/ReleaseNotes

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+utf8 cpp library
+Release 2.3.4
+
+A minor bug fix release. Thanks to all who reported bugs. 
+
+Note: Version 2.3.3 contained a regression, and therefore was removed.
+
+Changes from version 2.3.2
+- Bug fix [39]: checked.h Line 273 and unchecked.h Line 182 have an extra ';'
+- Bug fix [36]: replace_invalid() only works with back_inserter
+
+Files included in the release: utf8.h, core.h, checked.h, unchecked.h, utf8cpp.html, ReleaseNotes