gtk3: glitches because yours truly cannot tell x from y

(cherry picked from commit 2ca429e7)

Change-Id: I3b8620a4d8bffeb5e0558c35cb3a063238c12ba5

vcl/headless/svpgdi.cxx

@@ -156,12 +156,12 @@ namespace
         double x1, y1, x2, y2;
 
         cairo_clip_extents(cr, &x1, &y1, &x2, &y2);
-        extents.x = x1, extents.y = x2, extents.width = x2-x1, extents.height = y2-y1;
+        extents.x = x1, extents.y = y1, extents.width = x2-x1, extents.height = y2-y1;
 #if CAIRO_VERSION_MAJOR > 1 || (CAIRO_VERSION_MAJOR == 1 && CAIRO_VERSION_MINOR >= 10)
         cairo_region_t *region = cairo_region_create_rectangle(&extents);
 
         cairo_fill_extents(cr, &x1, &y1, &x2, &y2);
-        extents.x = x1, extents.y = x2, extents.width = x2-x1, extents.height = y2-y1;
+        extents.x = x1, extents.y = y1, extents.width = x2-x1, extents.height = y2-y1;
         cairo_region_intersect_rectangle(region, &extents);
 
         cairo_region_get_extents(region, &extents);