WebGLRenderer: Add "target" argument to "getClearColor"

[gkjohnson]

Related issue: #20819

Description

When scene.background is set to a color the renderer clear color is modified because WebGLRenderer.getClearColor() returns a reference to its color instance.

Repro (can be run in console on docs page):

renderer = new THREE.WebGLRenderer();
scene = new THREE.Scene();
scene.background = new THREE.Color( 0xff0000 );

console.log( renderer.getClearColor() );
generator = new THREE.PMREMGenerator( renderer );
generator.fromScene( scene );
console.log( renderer.getClearColor() );

In dev this logs 0, 0, 0 then 1, 0, 0. With this PR both log statements print 0, 0, 0 as expected.

[Mugen87]

The fix is correct since the same code style is used in various post processing passes, too.

@mrdoob What do you think about rewriting getClearColor() similar to getViewport() or getScissor().

this.getClearColor = function ( target ) {

    if ( target === undefined ) {

        console.warn( 'WebGLRenderer: .getClearColor() now requires a Color as an argument' );

        target = new Color();

    }

    return target.copy( background.getClearColor() );

};

However, this style requires some refactoring in the code base.

[mrdoob]

@mrdoob What do you think about rewriting getClearColor() similar to getViewport() or getScissor().

Sounds good to me!

Would you like @gkjohnson to do that in this PR? Or should we leave it for a different PR?

[Mugen87]

Or should we leave it for a different PR?

This one. In this way, the code base can migrated in one go.

[gkjohnson]

Done! Updated WebGLRenderer, docs, and examples.

I didn't touch WebGPURenderer but maybe that should be updated, too?

[mrdoob]

Damn, this escalated quickly 😁

[gkjohnson]

Damn, this escalated quickly

Heh, it's what you wanted, right?! 😁

[mrdoob]

Thanks!