monitor: Add mode spec helper checking resolution similarness

If two modes are roughly the same, they should probably use the same UI
scaling factor. I.e. for the same monitor, if a 4K mode was configured to
have a certain scaling factor, and we generate a new configuration with
a similar sized 4K mode, we should re-use the scale previously
configured; however if we e.g. go from a 4K mode to a FHD mode, we
shouldn't.

This allows implementing better hueristics when using the switch-config
feature, where we'd be less likely to loose the for a certain monitor
mode combination previously configured scaling factor.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2479>

src/backends/meta-monitor.c

@@ -1542,6 +1542,24 @@ meta_monitor_get_mode_from_id (MetaMonitor *monitor,
   return g_hash_table_lookup (priv->mode_ids, monitor_mode_id);
 }
 
+gboolean
+meta_monitor_mode_spec_has_similar_size (MetaMonitorModeSpec *monitor_mode_spec,
+                                         MetaMonitorModeSpec *other_monitor_mode_spec)
+{
+  const float target_ratio = 1.0;
+  /* The a size difference of 15% means e.g. 4K modes matches other 4K modes,
+   * FHD (2K) modes other FHD modes, and HD modes other HD modes, but not each
+   * other.
+   */
+  const float epsilon = 0.15;
+
+  return G_APPROX_VALUE (((float) monitor_mode_spec->width /
+                          other_monitor_mode_spec->width) *
+                         ((float) monitor_mode_spec->height /
+                          other_monitor_mode_spec->height),
+                         target_ratio, epsilon);
+}
+
 static gboolean
 meta_monitor_mode_spec_equals (MetaMonitorModeSpec *monitor_mode_spec,
                                MetaMonitorModeSpec *other_monitor_mode_spec)