This fixes the missing-invalidation problem outlined in commit
22023b10 (vcl lok: fix missing paints
due to zero-sized windows, 2016-07-12) also in Writer, not only in
Impress. Starting text edit, doing a change, and ending text edit now
results in the expected invalidations.

Ignoring visibility seems to be a better fix for the missing paints.
This way the headless case doesn't hit the corner cases of 0x0-sized
windows. Also, the gtk vclplug's GetClientSize() only returns 0x0 in
case the underlying window is disposed or it's minimized, but it does
hand out the size before Show() is called, so now the headless backend
is in sync with that.

Change-Id: I78698cbfce954c8c593d279ab057a87cfbe87260
Reviewed-on: https://gerrit.libreoffice.org/27224Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>