REG: Fix character size when exporting PDF on Windows

In Qt 4, there was a special case in qpdf.cpp which would
use the tmHeight from TEXTMETRIC to calculate the height
of the text instead of the pixel size. This was removed when
all code inside Q_WS_WIN was removed from QtGui. The tmHeight
is defined to be the same as ascent + descent, so we reinsert
this code for Windows.

It could be that this code is okay cross-platform, since the
font height and the pixel size are supposed to be the same, though
for some reason not on Windows. However, the safer approach is
to #ifdef it for Windows, since the bug is not present on
other platforms, and since it was #ifdef'd in Qt 4.

Task-number: QTBUG-30875
Change-Id: If0817768bf0ca5ce216842119422a0be944df0bf
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>