2 Point Perspective Grid Assistant thing

This assistant implements what is essentially a 2 point perspective grid with a "fixed" center of vision. It has 3 handles instead of 2. The first 2 handles are the vanishing points, and the 3rd handle helps define where the center of vision is (just to be clear: this means the 3rd handle isn't the literal cov).

The idea is this:

• perspective distortion gets stronger the farther an object is from the viewer's center of vision
• perspective distortion in an illustration improves the sense of depth and immersion
• having a visual guide of the distortion on the canvas is helpful for the illustrator
• having vanishing points automagically move together to keep perspective distortion consistent is helpful for the illustrator

To that end, this assistant keeps both vanishing points at a sufficient distance from each other, so the distortion level remains constant all over the canvas. And it draws a helpful square grid so the user can see exactly how distorted parts of their canvas are. ie they can see that a square on the floor at the periphery of their canvas is more distorted than a square in the center of vision.

Lots of work to done still:

• Snapping that works properly
• Grid drawing code that isn't unreadable/slow and looks good
• Integrate with brush's Perspective sensor
• Get suggestions from interested users, refine for possible use cases

It builds so far, and is listed as the "Conjugate" assistant in the Assistant Tool. "Conjugate" is a working name I took from the outdated linear perspective concept of a "conjugate pair of vanishing points", that is: vanishing points for a pair of lines that are perpendicular to each other.