Add GdkDrawingContext

Instead of giving out Cairo contexts, GdkWindow should provide a
"drawing context", which can then create Cairo contexts on demand; this
allows us to future proof the API for when we're going to use a
different rendering pipeline, like OpenGL.

https://bugzilla.gnome.org/show_bug.cgi?id=766675

gdk/gdkdrawingcontextprivate.h

@@ -0,0 +1,29 @@
+#ifndef __GDK_DRAWING_CONTEXT_PRIVATE_H__
+#define __GDK_DRAWING_CONTEXT_PRIVATE_H__
+
+#include "gdkdrawingcontext.h"
+
+G_BEGIN_DECLS
+
+#define GDK_DRAWING_CONTEXT_CLASS(klass)        (G_TYPE_CHECK_CLASS_CAST ((klass), GDK_TYPE_DRAWING_CONTEXT, GdkDrawingContextClass))
+#define GDK_IS_DRAWING_CONTEXT_CLASS(klass)     (G_TYPE_CHECK_CLASS_TYPE ((klass), GDK_TYPE_DRAWING_CONTEXT))
+#define GDK_DRAWING_CONTEXT_GET_CLASS(obj)      (G_TYPE_INSTANCE_GET_CLASS ((obj), GDK_TYPE_DRAWING_CONTEXT, GdkDrawingContextClass))
+
+struct _GdkDrawingContext
+{
+  GObject parent_instance;
+
+  GdkWindow *window;
+
+  cairo_region_t *clip;
+  cairo_t *cr;
+};
+
+struct _GdkDrawingContextClass
+{
+  GObjectClass parent_instance;
+};
+
+G_END_DECLS
+
+#endif /* __GDK_DRAWING_CONTEXT_PRIVATE_H__ */