The problem here is that we never actually hit the maExportGraphics
cache in SvXMLGraphicHelper, even though we are passing the same image
down repeatedly.

There are two bugs here:

(1) BitmapEx::operator== does not return true if we instantiate 2
Graphic objects from the same XGraphic, so change it to use the more
expensive operator==. To mitigate the cost, move the expensive checks to
the bottom of the method.

(2) in order to use an object in std::unordered_map, the object must
implement an equality function and a hash function. If two objects are
equal THEY MUST have the same hash value. Using the Impl* as the hash
value does not satisfy that condition, so rather use the checksum, which
does.

After these fixes, the save time drops to less than a second.

Also make the checksum method look more like the operator== method,
and add a checksum calculation method for SVG data that more accurately
reflects the underlying SVG data.

Change-Id: I4ca0c7bee60b2efa6fe42301e582c7b278022b46
Reviewed-on: https://gerrit.libreoffice.org/72615
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>