CubeCamera: Orient cameras right-side-up.

[WestLangley]

Fixes #3702, based on this suggestion.

CubeCamera currently sets its cameras "upside down", by setting a negative up-axis prior to calling camera.lookAt().

three.js/src/cameras/CubeCamera.js

Lines 17 to 20 in 6671e7c

	const cameraPX = new PerspectiveCamera( fov, aspect, near, far );
	cameraPX.layers = this.layers;
	cameraPX.up.set( 0, - 1, 0 );
	cameraPX.lookAt( 1, 0, 0 );

Doing so results in a mirroring of the rendered output in both the x-axis and y-axis.

This mirroring is required to accommodate the WebGL texture cube handedness and texture-origin conventions.

However, the same mirroring effect can be achieved by setting a negative fov.

The advantage of applying a negative fov instead of rotating the camera is that sprites rendered by a CubeCamera will now render right-side-up.

Sprites which have rotation of zero always render right-side-up in camera space.

//

Note: There will always be artifacts when a sprite straddles an edge of the top or bottom cube face. This is unavoidable.