tr2/lara/state: fix surface to swim input check

Before the input refactor, the check was (g_Input & LS_BACK) - LS_BACK
is a state, which happens to have the same value as IN_JUMP. So in the
refactor, it was converted to g_Input.back.

Resolves #1863.

docs/tr2/CHANGELOG.md

@@ -1,6 +1,7 @@
 ## [Unreleased](https://github.com/LostArtefacts/TRX/compare/tr2-0.6...develop) - ××××-××-××
 - added support for custom levels to enforce values for any config setting (#1846)
 - fixed depth problems when drawing certain rooms (#1853, regression from 0.6)
+- fixed being unable to go from surface swimming to underwater swimming without first stopping (#1863, regression from 0.6)
 
 ## [0.6](https://github.com/LostArtefacts/TRX/compare/tr2-0.5...tr2-0.6) - 2024-11-06
 - added a fly cheat key (#1642)

src/tr2/game/lara/state.c

@@ -909,7 +909,7 @@ void __cdecl Lara_State_SurfSwim(ITEM *item, COLL_INFO *coll)
     } else if (g_Input.right) {
         item->rot.y += LARA_SLOW_TURN;
     }
-    if (!g_Input.forward || g_Input.back) {
+    if (!g_Input.forward || g_Input.jump) {
         item->goal_anim_state = LS_SURF_TREAD;
     }
     item->fall_speed += 8;