Entity: properly account for upwards motion when calculating fall distance (#3867)

Previously, upwards movement wouldn't be considered, but downwards would, so if an entity bobbed up and down in the air for a while (e.g. while being comboed in PvP), the downwards distance would accumulate and deal a large amount of fall damage on touchdown.
This commit changes fall distance measurement to correctly account for upwards movement.

A better way of measuring fall distance would simply be to record the highest Y coordinate reached while in the air, and then measure the distance between that and the point of contact when landing. This would also remove the need to constantly update the fallDistance field. However, this would involve a BC break and will therefore have to wait until PM4.

src/pocketmine/entity/Entity.php

@@ -1451,8 +1451,14 @@ abstract class Entity extends Location implements Metadatable, EntityIds{
 				$this->fall($this->fallDistance);
 				$this->resetFallDistance();
 			}
-		}elseif($distanceThisTick < 0){
+		}elseif($distanceThisTick < $this->fallDistance){
+			//we've fallen some distance (distanceThisTick is negative)
+			//or we ascended back towards where fall distance was measured from initially (distanceThisTick is positive but less than existing fallDistance)
 			$this->fallDistance -= $distanceThisTick;
+		}else{
+			//we ascended past the apex where fall distance was originally being measured from
+			//reset it so it will be measured starting from the new, higher position
+			$this->fallDistance = 0;
 		}
 	}