Add shorthand flag in SlxiSubChunkFlags for XSCF_IGNORABLE_UNKNOWN | XSCF_IGNORABLE_VERSION.

src/saveload/extended_ver_sl.h

@@ -75,6 +75,8 @@ enum SlxiSubChunkFlags {
 	XSCF_IGNORABLE_VERSION        = 1 << 1,  ///< the loader is free to ignore this without aborting the load if the version is greater than the maximum that can be loaded
 	XSCF_EXTRA_DATA_PRESENT       = 1 << 2,  ///< extra data field is present, extra data in some sub-chunk/feature specific format, not used for anything yet
 	XSCF_CHUNK_ID_LIST_PRESENT    = 1 << 3,  ///< chunk ID list field is present, list of chunks which this sub-chunk/feature adds to the save game, this can be used to discard the chunks if the feature is unknown
+
+	XSCF_IGNORABLE_ALL            = XSCF_IGNORABLE_UNKNOWN | XSCF_IGNORABLE_VERSION, ///< all "ignorable" flags
 };
 DECLARE_ENUM_AS_BIT_SET(SlxiSubChunkFlags)