use p for identifers for points instead of x
use p for identifers for points instead of x

Fixes #61

sksullivan/shader-school,stackgl/shader-school,zhakui/shader-school,stackgl/shader-school,zhakui/shader-school,Heart2009/shader-school,Heart2009/shader-school,sksullivan/shader-school,sksullivan/shader-school