tr2/savegame: implement BSON saves

This implements BSON save games, similar to TR1, and opts for it to be
used in place of legacy for writing. Saves will also be stored in a
separate saves directory.

data/tr2/ship/cfg/TR2X_gameflow.json5

@@ -4,6 +4,7 @@
 
     "main_menu_picture": "data/images/title_eu.png",
     "savegame_fmt_legacy": "savegame.%d",
+    "savegame_fmt_bson": "save_tr2_%02d.dat",
 
     "cmd_init":           {"action": "exit_to_title"},
     "cmd_title":          {"action": "noop"},

data/tr2/ship/cfg/TR2X_gameflow_gm.json5

@@ -4,6 +4,7 @@
 
     "main_menu_picture": "data/images/title_eu_gm.png",
     "savegame_fmt_legacy": "savegame_gm.%d",
+    "savegame_fmt_bson": "save_trgm_%02d.dat",
 
     "cmd_init":           {"action": "exit_to_title"},
     "cmd_title":          {"action": "noop"},

data/tr2/ship/cfg/TR2X_gameflow_level.json5

@@ -2,7 +2,8 @@
     // This file is used to enable the -l argument support.
 
     "main_menu_picture": "data/images/title_eu.png",
-    "savegame_fmt_legacy": "savegame.%d",
+    "savegame_fmt_legacy": "savegame_custom.%d",
+    "savegame_fmt_bson": "save_tr2_custom_%02d.dat",
 
     "cmd_init":           {"action": "exit_to_title"},
     "cmd_title":          {"action": "noop"},