Equality test also needs to check if disjunct BitmapDevice instances
might not actually share the same memory buffer.
Change-Id: I09a93cb092a0039353be211ed053e991e7fe66f0
see https://gerrit.libreoffice.org/#/c/3367/
and Change-Id: I00c96fa77d04b33a6f8c8cd3490dfcd9bdc9e84a for details
Change-Id: I199a75bc4042af20817265d5ef85b1134a96ff5a