Fix moving platform behavior

It seems like when you pass an up_vector to move_and_slide, that handles moving platforms for you! Although not 100% correctly...

The manual implementation was added again without an up_vector and that seems to work perfectly now.

Player.gd

@@ -20,6 +20,7 @@ export var jump_exponent = 0.05
 export var almost_on_ground_length = 1.0
 
 var current_target_velocity := Vector3.ZERO
+var has_inherited_velocity := false
 
 export(NodePath) var solar_system
 
@@ -84,6 +85,27 @@ func is_on_ground():
 	return false
 
 
+# Returns true if the player is (almost) on the ground and that ground is a moving platform.
+func is_on_moving_platform():
+	return is_on_ground() and $GroundCheckRay.get_collider().is_in_group("MovingPlatform")
+
+
+# Set the current target velocity to the moving platform that is currently below the player.
+# If this was the first collision frame, also inherit the platform's velocity.
+func apply_moving_platform_velocity():
+	if not has_inherited_velocity:
+		velocity += $GroundCheckRay.get_collider().velocity
+		has_inherited_velocity = true
+	
+	current_target_velocity = $GroundCheckRay.get_collider().velocity
+
+
+# Reset the current target velocity and other variables relevant for moving platforms.
+func reset_moving_platform_velocity():
+	current_target_velocity = Vector3.ZERO
+	has_inherited_velocity = false
+
+
 # Called every frame. 'delta' is the elapsed time since the previous frame.
 func _physics_process(delta):
 	var move_velocity := Vector3.ZERO
@@ -105,6 +127,8 @@ func _physics_process(delta):
 	# Get acceleration caused by gravity
 	var gravity_acceleration = get_gravity_acceleration()
 	
+	# FIXME: Consider setting the gravity_acceleration to 0 if on ground
+	
 	# Apply both acceleration types
 	apply_acceleration((move_acceleration + gravity_acceleration) * delta)
 	
@@ -119,8 +143,14 @@ func _physics_process(delta):
 		velocity += -gravity_acceleration.normalized() * e_section
 		time_since_jump_start += delta
 	
+	# Check for moving platforms and lerp towards that
+	if is_on_moving_platform():
+		apply_moving_platform_velocity()
+	else:
+		reset_moving_platform_velocity()
+	
 	# Apply movement to position
-	velocity = move_and_slide(velocity, -gravity_acceleration.normalized(), true)
+	velocity = move_and_slide(velocity)
 	
 	# Clamp the velocity just to be save
 	velocity.x = clamp(velocity.x, -MAX_VEL, MAX_VEL)