[gtk+] gtk-demo: Fix drawing of gestures demo feedback on CSD windows

[Carlos Garnacho <carlosg src gnome org>]

commit d9a738dface57998f3c086cec17b08fc92189f55
Author: Carlos Garnacho <carlosg gnome org>
Date:   Tue Sep 15 13:19:31 2015 +0200

    gtk-demo: Fix drawing of gestures demo feedback on CSD windows
    
    It was wiping the translation initially applied to the cairo_t, so
    pick that up first before applying our own matrix changes.

 demos/gtk-demo/gestures.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)
---
diff --git a/demos/gtk-demo/gestures.c b/demos/gtk-demo/gestures.c
index b5a2958..f8e4758 100644
--- a/demos/gtk-demo/gestures.c
+++ b/demos/gtk-demo/gestures.c
@@ -86,9 +86,10 @@ drawing_area_draw (GtkWidget *widget,
       cairo_matrix_t matrix;
       gdouble angle, scale;
 
-      cairo_matrix_init_translate (&matrix,
-                                   allocation.width / 2,
-                                   allocation.height / 2);
+      cairo_get_matrix (cr, &matrix);
+      cairo_matrix_translate (&matrix,
+                              allocation.width / 2,
+                              allocation.height / 2);
 
       cairo_save (cr);