title: How to change javascript canvas DPI
body: <p>I'm creating a 2D html5 game, and I want to add a range slider who can decrease the canvas DPI to increase performances.</p>
  <p>Does anyone know how can I change the canvas DPI, but save the same canvas shape and pictue size ? (dont need to change the size of already drawed shapes, because I clear all and redraw at every frame)</p>