add "natural" coordinates

[Pomax]

The canvas uses (0,0) as upper left, instead of lower-left, with +Y running down, instead of up.

This is dumb, let's add some code that changes that (while giving people control over whether or not to use that).

[Pomax]

Note that this will almost certainly involve something like:

let hy = __ natural_coordinates ? screenToWorld(0, height).y : false;
someFunction(x, hy ? hy - y : y, ...)

rewrites to everything