fix: don't allow treeform over deep water

80ed8d6 added a check to allow transformations over dangerous
terrain when the player has tempflight, but this missed an interaction
with treeform. Treeform disables all sources of flight immediately, so
if the player had tempflight (e.g. via !flight), this would lead to
sequences like the following:

    You turn into a tree. Your roots penetrate the ground.
    You fall into the water!
    You can't survive in this terrain! You fly above the water, but the process is
    draining.
    Splash!
    You fall into the deep water. You sink to the bottom.

For the duration the player is in deep water, they get emergency flight
draining. (Permaflight had a correct version of this check.)  To fix,
move the tempflight check into `_flying_in_new_form` so the interaction
with forms forbidding flight is handled correctly.

Also, remove an obsolete Xom death check that looked at deep water/lava.

Author: rawlins

crawl-ref/source/ability.cc

@@ -1478,8 +1478,7 @@ static bool _check_ability_possible(const ability_def& abil, bool quiet = false)
     // dangerous.)
     if (abil.ability == ABIL_END_TRANSFORMATION)
     {
-        if (feat_dangerous_for_form(transformation::none, env.grid(you.pos()))
-            && !you.duration[DUR_FLIGHT])
+        if (feat_dangerous_for_form(transformation::none, env.grid(you.pos())))
         {
             if (!quiet)
             {

crawl-ref/source/transform.cc

@@ -1463,6 +1463,10 @@ static bool _flying_in_new_form(transformation which_trans)
     if (!you.duration[DUR_FLIGHT] && !you.attribute[ATTR_PERM_FLIGHT])
         return false;
 
+    // tempflight (e.g. from potion) enabled, no need for equip check
+    if (you.duration[DUR_FLIGHT])
+        return true;
+
     // Finally, do the calculation about what would be melded: are there equip
     // sources left?
     int sources = you.equip_flight();
@@ -1535,7 +1539,7 @@ static bool _transformation_is_safe(transformation which_trans,
                                     dungeon_feature_type feat,
                                     string *fail_reason)
 {
-    if (!feat_dangerous_for_form(which_trans, feat) || you.duration[DUR_FLIGHT])
+    if (!feat_dangerous_for_form(which_trans, feat))
         return true;
 
     if (fail_reason)

crawl-ref/source/xom.cc

@@ -920,7 +920,6 @@ static monster_type _xom_random_demon(int sever)
 static bool _player_is_dead()
 {
     return you.hp <= 0
-        || is_feat_dangerous(env.grid(you.pos()))
         || you.did_escape_death();
 }