monitor: Add helper to parse simple mode strings

This will be used to extract the resolution and refresh rate from
strings like "1920x1080@60.0" or "1280x720". This aims to replace the
use of the locale dependent sscanf() function.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2902>

src/backends/meta-monitor.h

@@ -318,4 +318,11 @@ gboolean meta_monitor_set_hdr_metadata (MetaMonitor            *monitor,
                                         MetaOutputHdrMetadata  *metadata,
                                         GError                **error);
 
+META_EXPORT_TEST
+gboolean meta_parse_monitor_mode (const char *string,
+                                  int        *out_width,
+                                  int        *out_height,
+                                  float      *out_refresh_rate,
+                                  float       fallback_refresh_rate);
+
 #endif /* META_MONITOR_H */