Physical "screen" size reported by the X11 backend is same as pixel count, fixing dpi to 25.4

Yeah, we should simply get the pixel density from the default screen..

Fix committed into lp:mir at revision None, scheduled for release in mir, milestone 0.24.0

This bug was fixed in the package mir - 0.24.0+16.10.20160815.3-0ubuntu1

---------------
mir (0.24.0+16.10.20160815.3-0ubuntu1) yakkety; urgency=medium

  * New upstream release 0.24.0 (https://launchpad.net/mir/+milestone/0.24.0)
    - ABI summary:
      . mirclient ABI unchanged at 9
      . mirserver ABI bumped to 41
      . mircommon ABI unchanged at 6
      . mirplatform ABI bumped to 12
      . mirprotobuf ABI unchanged at 3
      . mirplatformgraphics ABI bumped to 10
      . mirclientplatform ABI unchanged at 5
      . mirinputplatform ABI unchanged at 5
    - Enhancements:
      . Added a new tool 'mirvanity' which can use a high speed video camera
        to measure client-to-screen latency.
      . Added (build-time) support for desktop full OpenGL servers (disabled
        by default in favor of GLESv2).
      . Introduced new buffer semantics (NBS) and enabled it by default.
      . Avoided using libmirserver in mir_demo_standalone_render_overlays,
        making bringup traces less cluttered.
      . Removed TilingWindowManager from example servers.
      . Added two new mir_*_performance_test tools that are run as part of
        'mir_performance_tests'.
      . Added pointer confinement to the client API.
      . Added new platform supporting software EGLStreams.
      . Added an input platform that the nested server can use.
      . Allow configuration of the application_not_responding_detector
      . Handle server-side keymapping using XKBMapper.
      . Remove the offscreen display.
      . Add callback option to notify shells that the server is about to stop.
      . Add logging for ANativeWindow events on Android.
    - Bugs fixed:
      . usage of std:call_once in mirclient causes TLS collisions with some
        android devices (LP: #1599867)
      . AltGr not working on external keyboards (LP: #1565236)
      . [regression] unity8 fails to start when built with 0.24 series (lp:mir)
        (LP: #1597717)
      . CI failure in MirSurfaceVisibilityEvent.exposed_received_when_surface_
        raised_over_occluding_surface [called twice - over-saturated and active]
        (LP: #1556045)
      . package-built mir_demo_server does not start on device (LP: #1577357)
      . During surface creation, first stream in spec becomes default stream.
        (LP: #1577967)
      . mouse is getting stuck on a phantom edge (LP: #1580774)
      . [testsfail] in MirSurfaceVisibilityEvent.exposed_received_when_surface_
        raised_over_occluding_surface [never called] (LP: #1581385)
      . [enhancement] Add support for full OpenGL compositing (LP: #1420581)
      . [testsfail] ServerSignal.terminate_handler_is_called_for_SIGINT|SIGTERM
        (LP: #1570353)
      . [testsfail] ClientLatency.triple_buffered_client_has_less_than_two_
        frames_latency (LP: #1576690)
      . [testsfail] NestedServer.when_monitor_plugged_in_client_is_notified_of_
        new_display_configuration (LP: #1576760)
      . mir server crashed in what(): drmModeMoveCursor failed (returned -13)
        (LP: #1579630)
      . mirtest-dev is hard to use as the objects used are compiled with LTO
        (LP: #1583536)
      . [testsfail] ClientLogging.repo...