cache: use outline-bitmap hierarchy to slim down bitmap hash key

cache: use outline-bitmap hierarchy to slim down bitmap hash key

The bitmap hash key duplicated a lot of information the glyph hash
key already saves. The subclassing introduced recently complicates
this even more. Modify the hash key to utilize a pointer to the glyph
hash value instead, which is faster and more flexible. Make sure to
always empty the bitmap cache when the glyph cache is emptied.