Limited support of metadata import is added to RWGltf_GltfJsonParser.
Following Json data types are currently supported: int32, double, string,
array of int32, array of double, array of strings, Json object.
Notable unsupported types are: binary data, array of Json objects.
Metadata is processed for nodes and meshes.
Tests "gltf_export" are updated with import testing and renamed to "gltf".
Method RWGltf_GltfJsonParser::gltfParseSceneNode() is slightly
refactored: parsing of transformations and transformation matrices is
moved into separate functions.
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