glTF exporter: add filter hook for user extensions.

You can change filter tag to change default filtering

io_scene_gltf2/__init__.py

@@ -4,7 +4,7 @@
 bl_info = {
     'name': 'glTF 2.0 format',
     'author': 'Julien Duroure, Scurest, Norbert Nopper, Urs Hanselmann, Moritz Becher, Benjamin Schmithüsen, Jim Eckerlein, and many external contributors',
-    "version": (3, 2, 11),
+    "version": (3, 2, 12),
     'blender': (3, 1, 0),
     'location': 'File > Import-Export',
     'description': 'Import-Export as glTF 2.0',

io_scene_gltf2/blender/exp/gltf2_blender_gather_tree.py

@@ -5,6 +5,7 @@ import bpy
 import uuid
 
 from . import gltf2_blender_export_keys
+from io_scene_gltf2.io.exp.gltf2_io_user_extensions import export_user_extensions
 from mathutils import Quaternion, Matrix
 
 class VExportNode:
@@ -255,6 +256,7 @@ class VExportTree:
 
     def filter(self):
         self.filter_tag()
+        export_user_extensions('gather_tree_filter_tag_hook', self.export_settings, self)
         self.filter_perform()