Fix GLTFExporter MAT3 accessor support (#25426)

examples/jsm/exporters/GLTFExporter.js

@@ -1095,6 +1095,7 @@ class GLTFWriter {
 			2: 'VEC2',
 			3: 'VEC3',
 			4: 'VEC4',
+			9: 'MAT3',
 			16: 'MAT4'
 
 		};