Change window coordinate origin to the top left.

[bagnell]

The window coordinate origin is the bottom left in WebGL and it is the top left in the browser. Most users will expect any window coordinates to be in respect to the top left. Any functions that accept or return window coordinates should treat the top left of the window as the origin.

[pjcozzi]

@bagnell can we close this? Did we make this changes throughout already?

[bagnell]

@pjcozzi No. The origin is still the bottom left.