backends: Add method to get extents from viewport info

This way we know the stage extents without poking the stage.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>

src/backends/meta-viewport-info.c

@@ -187,3 +187,28 @@ meta_viewport_info_get_num_views (MetaViewportInfo *info)
 {
   return info->views->len;
 }
+
+void
+meta_viewport_info_get_extents (MetaViewportInfo *viewport_info,
+                                float            *width,
+                                float            *height)
+{
+  int min_x = G_MAXINT, min_y = G_MAXINT, max_x = G_MININT, max_y = G_MININT, i;
+
+  g_return_if_fail (viewport_info != NULL);
+
+  for (i = 0; i < viewport_info->views->len; i++)
+    {
+      ViewInfo *info = &g_array_index (viewport_info->views, ViewInfo, i);
+
+      min_x = MIN (min_x, info->rect.x);
+      max_x = MAX (max_x, info->rect.x + info->rect.width);
+      min_y = MIN (min_y, info->rect.y);
+      max_y = MAX (max_y, info->rect.y + info->rect.height);
+    }
+
+  if (width)
+    *width = (float) max_x - min_x;
+  if (height)
+    *height = (float) max_y - min_y;
+}

src/backends/meta-viewport-info.h

@@ -51,4 +51,8 @@ int meta_viewport_info_get_neighbor (MetaViewportInfo     *info,
 
 int meta_viewport_info_get_num_views (MetaViewportInfo *info);
 
+void meta_viewport_info_get_extents (MetaViewportInfo *info,
+                                     float            *width,
+                                     float            *height);
+
 #endif /* META_VIEWPORT_INFO_H */