Use ordinary (not flipped) Cartesian coordinates in Turtle visualizer

[gwhitney]

Currently the Turtle visualizer uses "p5 coordinates" in which the positive x direction is right and the positive y direction is down.
In formula mode, these coordinates are confusing, and they mean that turns are clockwise rather than counterclockwise as the usual mathematical convention would have.

The steering committee decided that we should flip the positive y direction for Turtle.

[gwhitney]

Note that the picture being produced by the Zeta Zero #10143 visualization exactly matches the one on stack exchange so that must mean the current formulas have somewhere in them something like a sign error that is canceling out the current coordinate flip. So in this revision, the formula in this featured specimen will have to be fixed, to keep the picture the same. Most turtle pictures will flip.