Account for library when gathering image URI

When a material with a relative path to an image is linked from another
.blend file, that path should be relative to that .blend file instead of
relative to the current file.

addons/io_scene_gltf2/blender/exp/material/gltf2_blender_gather_image.py

@@ -56,7 +56,11 @@ def gather_image(
         uri, factor_uri = __gather_uri(image_data, mime_type, name, export_settings)
     else:
         # Retrieve URI relative to exported glTF files
-        uri = __gather_original_uri(image_data.original.filepath, export_settings)
+        uri = __gather_original_uri(
+            image_data.original.filepath,
+            blender_shader_sockets[0].socket.id_data.library,
+            export_settings
+        )
         # In case we can't retrieve image (for example packed images, with original moved)
         # We don't create invalid image without uri
         factor_uri = None
@@ -81,9 +85,9 @@ def gather_image(
     # We also return image_data, as it can be used to generate same file with another extension for WebP management
     return image, image_data, factor, None
 
-def __gather_original_uri(original_uri, export_settings):
+def __gather_original_uri(original_uri, library, export_settings):
+    path_to_image = bpy.path.abspath(original_uri, library=library)
 
-    path_to_image = bpy.path.abspath(original_uri)
     if not os.path.exists(path_to_image): return None
     try:
         rel_path = os.path.relpath(