The cache key is just the name of the cursor, so if a previously added
cursor had e.g. scale == 1, if we ask for a new cursor with scale == 2,
we might still fetch the scale == 1 cursor from the cache. Avoid this by
making sure the scale of the cached one is correct.

If it isn't, load the cursor as normal, and update the cache entry with
the new properly scaled cursor.

Fixes: #1183