Make WebGLRenderer.setViewport(), setScissor() relative to lower-left corner

[WestLangley]

Fixes #12210.

When specifying the viewport and the scissor box, (x, y) is the coordinate of the lower left corner of the rectangular region.

https://developer.mozilla.org/en-US/docs/Web/API/WebGLRenderingContext/viewport

[WestLangley]

Now, .setViewport() follows the webGL convention, so the viewport transform, which converts from NDC space to window space, is correct when applied to NDC coordinates.

Also, .setViewport() now follows the same convention as WebGLRenderTarget.viewport.set().

---

Aside: .getCurrentViewport() should correctly return the viewport previously set by .setViewport(). If it doesn't, that is a separate issue, unrelated to this PR.

[Mugen87]

Yeah, I also think it's better to stick to the WebGL conventions.

[WestLangley]

I have other PRs in the pipeline that depend on this correction.

/ping @Mugen87 I don't know what else I can do.

[Mugen87]

I'm fine with this change 😇

[mrdoob]

Thanks! 😅