X11: set fallback logical DPI to 96

Diego Violarequested to merge
dviola/qtbase:QTBUG-108602 into kde/5.15

Returning physical DPI from logicalDpi() is problematic, as explained in commit 77e04acb.

The most predictable implementation is to never return physical DPI from QPlaformScreen::logicalDpi(). Other platform plugins already does this, and this change brings xcb in line with the rest of Qt.

We have the QPlatformScreen::physicalSize() API which covers returning physical DPI (indirectly); Options for selecting which one to use can be implemented on top of these (see QT_USE_PHYSICAL_DPI).

Change-Id: Ifc41229fa63734a2eb06b3acefd97b2ed3e57c2d Reviewed-by: Tor Arne Vestbø tor.arne.vestbo@qt.io (cherry picked from commit 72381235)

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