Currently we create a "disabled" version of  bitmaps in "Image"
class (used for toolbar icons for example) in every draw call.
This is not really optimal. The solution is to calculate the
checksum of the original and create a "disabled" version only if
the checksum changes.

(cherry picked from commit 86186de4)

Conflicts:
	vcl/source/gdi/impimage.cxx

Change-Id: I6ed2d08c64fee028b479fedb35e4384f112e98e2
Reviewed-on: https://gerrit.libreoffice.org/19160Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>
(cherry picked from commit c34cb60b)