We calculate the scanline size wrongly when writing a PDF so the
image is not exported correctly. BitmapAccess already has a method
to determine the scanline size so use that instead.

Change-Id: Icccba8f26c5e0fd4bd4c37bba7c5a7fe8d0094bd
(cherry picked from commit 82e0c38e)
Signed-off-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/20595Tested-by: Jenkins <ci@libreoffice.org>