X11: set fallback logical DPI to 96

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>

src/plugins/platforms/xcb/qxcbscreen.cpp

@@ -690,12 +690,12 @@ QDpi QXcbScreen::logicalDpi() const
     if (forcedDpi > 0)
         return QDpi(forcedDpi, forcedDpi);
 
-    // Fall back to physical virtual desktop DPI, but prevent
-    // using DPI values lower than 96. This ensuers that connecting
-    // to e.g. a TV works somewhat predictabilly.
-    QDpi virtualDesktopPhysicalDPi = m_virtualDesktop->dpi();
-    return QDpi(std::max(virtualDesktopPhysicalDPi.first, 96.0),
-                std::max(virtualDesktopPhysicalDPi.second, 96.0));
+    // Fall back to 96 DPI in case no logical DPI is set. We don't want to
+    // return physical DPI here, since that is a differnt type of DPI: Logical
+    // DPI typically accounts for user preference and viewing distance, and is
+    // quantized into DPI classes (96, 144, 192, etc); pysical DPI is an exact
+    // physical measure.
+    return QDpi(96, 96);
 }
 
 QPlatformCursor *QXcbScreen::cursor() const