GTK3 HiDPI: disable window scaling and get display DPI via GTK3

Change-Id: I66b92146757352408a331f9e23289839f443ff79
(cherry picked from commit 2a19bb85)
Reviewed-on: https://gerrit.libreoffice.org/17553Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

vcl/inc/unx/gtk/gtkgdi.hxx

@@ -61,6 +61,7 @@ public:
 
     cairo_t* getCairoContext() const;
 
+    virtual void GetResolution(sal_Int32& rDPIX, sal_Int32& rDPIY) SAL_OVERRIDE;
 private:
     GtkWidget       *mpWindow;
     static GtkStyleContext *mpButtonStyle;

vcl/unx/gtk/app/gtkdata.cxx

@@ -46,6 +46,12 @@
 
 #include <vcl/svapp.hxx>
 
+#if GTK_CHECK_VERSION(3,10,0)
+#ifdef GDK_WINDOWING_X11
+#include <gdk/gdkx.h>
+#endif
+#endif
+
 using namespace vcl_sal;
 
 /***************************************************************
@@ -87,6 +93,19 @@ GtkSalDisplay::GtkSalDisplay( GdkDisplay* pDisplay ) :
 #else
     m_bX11Display = true;
 #endif
+
+#if GTK_CHECK_VERSION(3,10,0)
+#ifdef GDK_WINDOWING_X11
+    if (m_bX11Display)
+    {
+        if (!getenv("GDK_SCALE"))
+        {
+            gdk_x11_display_set_window_scale(m_pGdkDisplay, 1);
+        }
+    }
+#endif
+#endif
+
 }
 
 GtkSalDisplay::~GtkSalDisplay()

vcl/unx/gtk3/gdi/gtk3salnativewidgets-gtk.cxx

@@ -2097,4 +2097,22 @@ cairo_t* GtkSalGraphics::getCairoContext() const
     return mpFrame->getCairoContext();
 }
 
+void GtkSalGraphics::GetResolution(sal_Int32& rDPIX, sal_Int32& rDPIY)
+{
+    GdkScreen* pScreen = gtk_widget_get_screen(mpWindow);
+    double fResolution = -1.0;
+    g_object_get(pScreen, "resolution", &fResolution, nullptr);
+
+    int nScaleFactor = 1;
+
+#if GTK_CHECK_VERSION(3, 10, 0)
+    nScaleFactor = gdk_window_get_scale_factor(widget_get_window(mpWindow));
+#endif
+
+    if (fResolution > 0.0)
+        rDPIX = rDPIY = sal_Int32(fResolution * nScaleFactor);
+    else
+        rDPIX = rDPIY = 96;
+}
+
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */