[gtk/gtk-3-24: 2/3] demo: Use pinch translation in the gestures demo

[Matthias Clasen <matthiasc src gnome org>]

commit 1c050bdcf4dbd75b8de8940d2af5fd1038b3bab9
Author: Yariv Barkan <nushoin users noreply github com>
Date:   Sat Dec 14 11:27:44 2019 +0200

    demo: Use pinch translation in the gestures demo
    
    Add the pinch translation offset to the pinch gesture demo. That allows
    moving the square around in addition to the already-supported rotate and
    zoom.

 demos/gtk-demo/gestures.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
---
diff --git a/demos/gtk-demo/gestures.c b/demos/gtk-demo/gestures.c
index 1e88a9ec91..9632e7d665 100644
--- a/demos/gtk-demo/gestures.c
+++ b/demos/gtk-demo/gestures.c
@@ -96,11 +96,12 @@ drawing_area_draw (GtkWidget *widget,
       cairo_pattern_t *pat;
       cairo_matrix_t matrix;
       gdouble angle, scale;
+      gdouble x_center, y_center;
+
+      gtk_gesture_get_bounding_box_center (GTK_GESTURE (zoom), &x_center, &y_center);
 
       cairo_get_matrix (cr, &matrix);
-      cairo_matrix_translate (&matrix,
-                              allocation.width / 2,
-                              allocation.height / 2);
+      cairo_matrix_translate (&matrix, x_center, y_center);
 
       cairo_save (cr);