Change the perspective camera's perspective origin

This would be identical to CSS perspective-origin.

For now, I’ll come up with something using camera frustum size and WebGLRenderer.setViewport.

Having the API built into perspective cameras would be nice.

Is it the same concept as the one with THREE.StereoCamera? It creates two cameras, for both eyes, and each camera is shifted and the projection is not symmetrical. So, it is a kind of changed perspective origin, I think.

See lines 57-89:

Ah yeah, that may be a hint for the math way of doing it instead of with setViewport. Thanks! Need to figure how to apply that based on a distance from the edge of the viewport.