X11: restrict fallback logical DPI to 96 and higher

QXcbScreen determines logical DPI using the following priority:

1) use Xft.dpi if set
2) virtual desktop size / virtual desktop physical size

(This change does not restrict or alter the value obtained from
Xft.dpi in any way)

The fallback option 2) has several problems:
- It is a physical DPI, which does not account for
  viewing distance or user preference
- It is a global value for the entire virtual desktop
  (not per-screen)
- X servers usually fake the virtual desktop physical
  size such that the computed DPI ends up at 96; in
  cases where this does not happen we can end up with
  a surprising DPI value.

We might be better off just hardcoding 96 here, to avoid
picking up an incorrect physical DPI when for example
connecting to a TV (as reported in QTBUG-67928).

This change does not go as far as hardcoding 96, but instead
restricts the (fallback) DPI value to be 96 or higher.

Pick-to: 5.15
Task-number: QTBUG-67928
Change-Id: Ieea6c7a137261282b40302fb4c19273c5def10af
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/xcb/qxcbscreen.cpp

@@ -698,7 +698,12 @@ QDpi QXcbScreen::logicalDpi() const
     if (forcedDpi > 0)
         return QDpi(forcedDpi, forcedDpi);
 
-    return m_virtualDesktop->dpi();
+    // 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));
 }
 
 QPlatformCursor *QXcbScreen::cursor() const