add function reporting X endpoint of a text box

uses the "print offscreen" trick

vacation.p8

@@ -178,6 +178,10 @@ function txtbox:draw()
  print(self.text, self.x, self.y, self.col)
 end
 
+function txtbox:xmax()
+ return print(self.text, self.x, -9999)
+end
+
 spring = {
  from = 128,
  to = 0,