Hi-DPI patches for 4.2

Hi Caolán, all,

I've just pushed a backport of the hi-dpi patches from master to gerrit
for libreoffice-4-2 integration - as was requested earlier, to fix the
unfortunate state of LibreOffice on the hi-dpi displays.  It is the
following 5 patches (order is important):

https://gerrit.libreoffice.org/#/c/8516/
https://gerrit.libreoffice.org/#/c/8517/
https://gerrit.libreoffice.org/#/c/8518/
https://gerrit.libreoffice.org/#/c/8519/
https://gerrit.libreoffice.org/#/c/8520/

Keith confirmed that they fix the hi-dpi issues he was seeing in
LibreOffice 4.2.

They are supposed to be safe for normal displays; that is anything
non-safe should be enclosed in an "if (mnDPIScaleFactor > 1)".  Few
cases make the computation a bit more general, like:

+    long yOffset = (aRect.GetHeight() - mpImpl->maImage.GetSizePixel().Height()) / 2;
+
     if( mpImpl->mnState == SIGNATURESTATE_SIGNATURES_OK )
     {
-        ++aRect.Top();
+        aRect.Top() += yOffset;

etc. but hopefully such places are safe enough too.

It is only 5 patches, as I have squashed the follow-up fixes into the
appropriate patches, but other than that, it should bring us to the very
same state that is in master.

Thank you in advance,
Kendy