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/GraphicsBitmap.cpp

@@ -0,0 +1,25 @@
+#include "GraphicsBitmap.h"
+#include <AK/kmalloc.h>
+
+RetainPtr<GraphicsBitmap> GraphicsBitmap::create(const Size& size)
+{
+    return adopt(*new GraphicsBitmap(size));
+}
+
+GraphicsBitmap::GraphicsBitmap(const Size& size)
+    : m_size(size)
+{
+    m_data = (byte*)kmalloc(size.width() * size.height() * 4);
+}
+
+GraphicsBitmap::~GraphicsBitmap()
+{
+    kfree(m_data);
+}
+
+dword* GraphicsBitmap::scanline(int y)
+{
+    unsigned pitch = m_size.width() * 4;
+    return (dword*)(((byte*)m_data) + (y * pitch));
+}
+