Platform: expose window/framebuffer size in *XApplication classes.

For consistency with the new Sdl2Application implementation. The
dpiScaling() function is not exposed, as I don't want to go through all
the pain of getting this through raw Xlib -- and the *XApplication
classes are meant to be used in very restricted embedded environments
that probably don't have any HiDPI issues anyway.

src/Magnum/Platform/AbstractXApplication.cpp

@@ -74,7 +74,7 @@ bool AbstractXApplication::tryCreate(const Configuration& configuration, const G
 
     CORRADE_ASSERT(_context->version() == GL::Version::None, "Platform::AbstractXApplication::tryCreate(): context already created", false);
 
-    _viewportSize = configuration.size();
+    _windowSize = configuration.size();
 
     /* Get default X display */
     _display = XOpenDisplay(nullptr);
@@ -154,8 +154,8 @@ int AbstractXApplication::exec() {
                 /* Window resizing */
                 case ConfigureNotify: {
                     Vector2i size(event.xconfigure.width, event.xconfigure.height);
-                    if(size != _viewportSize) {
-                        _viewportSize = size;
+                    if(size != _windowSize) {
+                        _windowSize = size;
                         viewportEvent(size);
                         _flags |= Flag::Redraw;
                     }

src/Magnum/Platform/AbstractXApplication.h

@@ -179,6 +179,23 @@ class AbstractXApplication {
 
         /** @{ @name Screen handling */
 
+    public:
+        /**
+         * @brief Window size
+         *
+         * Window size to which all input event coordinates can be related.
+         * Same as @ref framebufferSize().
+         */
+        Vector2i windowSize() const { return _windowSize; }
+
+        /**
+         * @brief Framebuffer size
+         *
+         * Size of the default framebuffer. Same as @ref windowSize().
+         */
+        Vector2i framebufferSize() const { return _windowSize; }
+
+    protected:
         /**
          * @brief Swap buffers
          *
@@ -251,7 +268,7 @@ class AbstractXApplication {
         std::unique_ptr<Platform::GLContext> _context;
 
         /** @todo Get this from the created window */
-        Vector2i _viewportSize;
+        Vector2i _windowSize;
 
         Flags _flags;
 };