cairo canvas: Line dashing size depends on the line width.

Change-Id: I6a3f563b6effd37b448ec3e8463a87879e0566d4
Reviewed-on: https://gerrit.libreoffice.org/6833
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>