libsysprof-gtk: move flip-y to SysprofXYLayer

So that we can use it to perform value coordinate flipping on multiple
charts and graphs.

src/libsysprof-gtk/sysprof-xy-layer-private.h

@@ -37,6 +37,8 @@ struct _SysprofXYLayer
 
   SysprofNormalizedSeries *normal_x;
   SysprofNormalizedSeries *normal_y;
+
+  guint                    flip_y : 1;
 };
 
 struct _SysprofXYLayerClass