Start refactoring graphics system to have per-window backing stores.

It was fun for everyone to share a single framebuffer but it was also
kinda really awful. Let's move towards having a "GraphicsBitmap" as the
backing store for each Window.

This is going to need a lot of refactoring so let's get started.

Widgets/Size.h

@@ -13,6 +13,12 @@ public:
     void setWidth(int w) { m_width = w; }
     void setHeight(int h) { m_height = h; }
 
+    bool operator==(const Size& other) const
+    {
+        return m_width == other.m_width &&
+               m_height == other.m_height;
+    }
+
 private:
     int m_width { 0 };
     int m_height { 0 };