Evict previous key->id mapping before updating the memory caches

Stale entries in the key->id hash were causing mismatching masks being returned from the cache. If a Framesvg F inserted a pixmap with key A, then inserting a pixmap with key B a lookup for A would return B. Because keysToCache would still contain A->F and looking up in the pixmap cache would find the pixmap corresponding to B for F.