Fix HDPI problem for WebGL examples

[ttnghia]

Hi there,

I create this issue to make sure we don't forget about our high-DPI problem: mouse position is not correct when running WebGL examples on high DPI monitor.

The examples still have this problem: Bullet and Fluid2D (please add more if you know).

[mosra]

Fixed in 5d76174 🎉

Sorry it took so long. I'll update the online versions together with a website update for the 2020.02 release.