Merge branch 'gtk-dpi'

A bunch of improvements for per-monitor DPI support in wxGTK and
wxGraphicsContext more generally.

Closes https://github.com/wxWidgets/wxWidgets/pull/995

interface/wx/display.h

@@ -26,6 +26,23 @@ public:
     */
     wxDisplay(unsigned int index = 0);
 
+    /**
+        Constructor creating the display object associated with the given
+        window.
+
+        This is the most convenient way of finding the display on which the
+        given window is shown while falling back to the default display if it
+        is not shown at all or positioned outside of any display.
+
+        @param window
+            A valid, i.e. non-null, window.
+
+        @see GetFromWindow()
+
+        @since 3.1.2
+     */
+    explicit wxDisplay(const wxWindow* window);
+
     /**
         Destructor.
     */

interface/wx/graphics.h

@@ -1095,6 +1095,19 @@ public:
     */
     virtual void GetDPI( wxDouble* dpiX, wxDouble* dpiY);
 
+    /**
+        Returns the associated window if any.
+
+        If this context was created using Create() overload taking wxWindow or
+        wxWindowDC, this method returns the corresponding window. Otherwise
+        returns @NULL.
+
+        @return A possibly @NULL window pointer.
+
+        @since 3.1.2
+     */
+    wxWindow* GetWindow() const;
+
     /** @}
     */