Using the DPI from Application::GetDefaultDevice() is a much better
idea, especially that now oox::GraphicHelper::GraphicHelper() and
oox::drawingml::DrawingML::WriteSrcRect() are in sync.

Should fix the testCropPixel() failure in CppunitTest_sw_ooxmlexport
that appears on HiDPI systems.

Also, fix all the rounding problems that now became visible when the DPI
is the same for both import and export.

Change-Id: Iceb34a8a5a1eaa8ce0824491521ad6b4d2f6949c
Reviewed-on: https://gerrit.libreoffice.org/14280Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>