snapshot: "Inherit" from GdkSnapshot

This is a neat trick to get around the circularity between GDK, GSK and
GTK that we inherit with the GdkPaintable interface.

GdkPaintable uses GtkSnapshot
GtkSnapshot creates GskRenderNodes
GskRenderNodes use GdkTextures
GdkTexture will soon implement GdkPaintable

This causes a loop that spans GDK, GSK and GTK and this is the easiest
way to resolve it without breaking bindings (at least that's the idea).

gtk/gtksnapshotprivate.h

@@ -80,7 +80,7 @@ struct _GtkSnapshotState {
   } data;
 };
 
-struct _GtkSnapshot {
+struct _GdkSnapshot {
   int ref_count;
   gboolean               record_names;
   GskRenderer           *renderer;

gtk/gtktypes.h

@@ -39,7 +39,7 @@ typedef struct _GtkClipboard	       GtkClipboard;
 typedef struct _GtkRequisition	       GtkRequisition;
 typedef struct _GtkSelectionData       GtkSelectionData;
 typedef struct _GtkSettings            GtkSettings;
-typedef struct _GtkSnapshot            GtkSnapshot;
+typedef GdkSnapshot                    GtkSnapshot;
 typedef struct _GtkStyleContext        GtkStyleContext;
 typedef struct _GtkTooltip             GtkTooltip;
 typedef struct _GtkWidget              GtkWidget;