Pressing a key in Writer used to lay out the relevant string 4 times
(counting GenericSalLayout::LayoutText() invocations), save one of them
by pre-calculating the layout and sharing it between GetTextArray() and
DrawTextArray().

Change-Id: Ic2194bdcbe94a546d57745463ae81e6b0cadcd92
Reviewed-on: https://gerrit.libreoffice.org/59208Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins