WebGLCubeRenderTarget: simplify constructor signature

[WestLangley]

Since width and height must be the same, this PR simplifies the constructor signature:

WebGLCubeRenderTarget( width, height, options )

is now

WebGLCubeRenderTarget( resolution, options ).

[mrdoob]

I'm unsure about resolution... Maybe size?
Is this a limitation of WebGL 2.0 too?

[WestLangley]

I'm unsure about resolution... Maybe size?

Done!

Is this a limitation of WebGL 2.0 too?

A cube has 6 identically-sized 2D faces. I am not aware of a WebGL 2.0 use case that is different.

[Mugen87]

I am not aware of a WebGL 2.0 use case that is different.

Maybe it's more safe to keep the signature as it is since it's more flexibel. How knows which use cases come up in the future....

[WestLangley]

Who knows which use cases come up in the future....

This is not about use cases. There are no other use cases allowed. If you pass in a height that is not equal to width, you get a gl error.

[Mugen87]

If you pass in a height that is not equal to width, you get a gl error.

Um, is this also true for WebGPU?

[WestLangley]

It is true everywhere: A cube has 6 square sides.

We should not have had both width and height parameters in the first place.

[WestLangley]

@mrdoob I honored your requested change.

[mrdoob]

Thanks!