Two potential glitches in font oversampling

Three minor things I came across while searching for the cause of https://github.com/godotengine/godot/issues/15392 that would be worth a second look:

• valid should become false in update_oversampling
• in update_oversampling a list is accessed that is mutexed on the other occasions, but not here
• also in update_oversampling, as emit_changed() is not deferred AFAIK, I wonder if this should be called from inside the iterator loop (in case the list changes); well, it's a doubly-linked list that should handle this gracefully, but still