T71495 describes two problems with animation of textures not showing up in
the dope sheet:
1. textures connected to force fields
2. textures of brushes
This patch resolves the first case.
An alternative would be to switch to iteration of dependencies using
`BKE_library_foreach_ID_link()`. This is a good idea to do at some point,
but adding these few lines was considerably easier & safer to do.
9b74e648c5