Change viewport plan save format

The previous format stored sparse keys in a non-sparse array chunk,
which resulted in a significant savegame size penalty.

src/saveload/extended_ver_sl.cpp

@@ -45,7 +45,7 @@ std::vector<uint32> _sl_xv_discardable_chunk_ids;           ///< list of chunks
 static const uint32 _sl_xv_slxi_chunk_version = 0;          ///< current version os SLXI chunk
 
 const SlxiSubChunkInfo _sl_xv_sub_chunk_infos[] = {
-	{ XSLFI_ENH_VIEWPORT_PLANS,     XSCF_IGNORABLE_ALL,     1,                                    1, "enh_viewport_plans",    NULL, NULL, "PLAN,PLLN" },
+	{ XSLFI_ENH_VIEWPORT_PLANS,     XSCF_IGNORABLE_ALL,       2,   2, "enh_viewport_plans",        NULL, NULL, "PLAN"      },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, NULL, NULL, NULL, NULL },// This is the end marker
 };