app: fix GimpMeter history-graph painting when clipped

In GimpMeter, use cairo_path_extents() for getting the history
graph's extents, rather than cairo_clip_extents(), since the latter
may also include clipping applied by GTK.

(cherry picked from commit b8e08cddbc)

app/widgets/gimpmeter.c

@@ -571,9 +571,8 @@ gimp_meter_expose_event (GtkWidget      *widget,
                      allocation.width - BORDER_WIDTH - 0.5 * size,
                      0.25 * size);
       cairo_close_path (cr);
-      cairo_clip (cr);
 
-      cairo_clip_extents (cr,
+      cairo_path_extents (cr,
                           &history_x1, &history_y1,
                           &history_x2, &history_y2);
 
@@ -582,6 +581,8 @@ gimp_meter_expose_event (GtkWidget      *widget,
       history_x2 = ceil  (history_x2);
       history_y2 = ceil  (history_y2);
 
+      cairo_clip (cr);
+
       /* paint history background */
       gdk_cairo_set_source_color (cr, &style->light[state]);
       cairo_paint (cr);