Fix broken-down aircraft stuck at 0 speed on load.

Bump improved breakdowns version.

src/saveload/afterload.cpp

@@ -2856,6 +2856,21 @@ bool AfterLoadGame()
 			}
 		}
 	}
+	if (!SlXvIsFeaturePresent(XSLFI_IMPROVED_BREAKDOWNS, 4)) {
+		Vehicle *v;
+		FOR_ALL_VEHICLES(v) {
+			switch(v->type) {
+				case VEH_AIRCRAFT:
+					if (v->breakdown_type == BREAKDOWN_AIRCRAFT_SPEED && v->breakdown_severity == 0) {
+						v->breakdown_severity = max(1, min(v->vcache.cached_max_speed >> 4, 255));
+					}
+					break;
+
+				default:
+					break;
+			}
+		}
+	}
 
 	/* The road owner of standard road stops was not properly accounted for. */
 	if (IsSavegameVersionBefore(172)) {

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_IMPROVED_BREAKDOWNS,    XSCF_NULL,                3,   3, "improved_breakdowns",       NULL, NULL, NULL        },
+	{ XSLFI_IMPROVED_BREAKDOWNS,    XSCF_NULL,                4,   4, "improved_breakdowns",       NULL, NULL, NULL        },
 	{ XSLFI_NULL, XSCF_NULL, 0, 0, NULL, NULL, NULL, NULL },// This is the end marker
 };