Fix regression: Escape from conveyor spikes

This fixes a regression from 2.3. Consider the following diagram:

       CC
    X  CC
      <<<<

"C" indicates one tile of a checkpoint entity, "X" indicates a spike
tile, and "<" indicates one tile of a conveyor entity that has the
default speed (4 pixels per frame) going leftwards.

Now consider if the player were to touch the checkpoint and die. In 2.2,
they would be able to escape from the spike by holding right. But in
2.3, they would not be able to, and would die from the spike
continuously with no escape.

This happens because in 2.2, the player would spawn a couple pixels off
the surface, and this gained them an extra frame of movement to move
away from the conveyor. 2.3 places the player directly on the ground,
moving them one frame earlier and thus forcing them to their doom.

Now consider the following diagram:

      CC
    X CC
     <<<<

The difference with the previous diagram is that this time, the spike is
one tile closer. This time, there is no escape in 2.2 and you will
always die no matter what.

By the way, both diagrams have the same behavior if the conveyor is
rightwards and if everything is flipped upside-down. Thankfully, it
doesn't seem to be direction-dependent.

The reason 2.3 lowered the player onto the surface was for consistency
(see PR #502), and I don't want to undo that. So I think the best
solution here is to re-add the extra frame of control by conveyors only
moving the player after lifeseq has advanced enough.

desktop_version/src/Logic.cpp

@@ -835,7 +835,7 @@ void gamelogic(void)
                 }
             }
 
-            if(obj.horplatforms)
+            if (obj.horplatforms)
             {
                 for (int ie = obj.entities.size() - 1; ie >= 0;  ie--)
                 {
@@ -859,15 +859,18 @@ void gamelogic(void)
                 //is the player standing on a moving platform?
                 int i = obj.getplayer();
                 float j = obj.entitycollideplatformfloor(i);
-                if (INBOUNDS_VEC(i, obj.entities) && j > -1000)
+                // To match 2.2, we need to give a bit of leeway in the lifeseq
+                // before we start pushing the player.
+                const bool horz_active = game.lifeseq < 8;
+                if (horz_active && INBOUNDS_VEC(i, obj.entities) && j > -1000)
                 {
                     obj.entities[i].newxp = obj.entities[i].xp + j;
                     obj.entitymapcollision(i);
                 }
                 else
                 {
                     j = obj.entitycollideplatformroof(i);
-                    if (INBOUNDS_VEC(i, obj.entities) && j > -1000)
+                    if (horz_active && INBOUNDS_VEC(i, obj.entities) && j > -1000)
                     {
                         obj.entities[i].newxp = obj.entities[i].xp + j;
                         obj.entitymapcollision(i);