In some large PDFs the two maps for
mapping GraphicsContext to ID and finding
GraphicsContext given and ID are huge (over
160 MB in one case). By using a bimap the
footprint went down to 110 MB.

Reviewed-on: https://gerrit.libreoffice.org/45380Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>
Tested-by: Ashod Nakashian <ashnakash@gmail.com>
(cherry picked from commit 551543bf)
(cherry picked from commit ac328157)

Change-Id: I59a444b5efa09fcabc26fa492de723ca063e14a4
Reviewed-on: https://gerrit.libreoffice.org/46988Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>