WebGLRenderTarget: Refactor copy().

[Mugen87]

Related issue: see https://github.com/mrdoob/three.js/compare/dev...Mugen87:dev62?expand=1

Description

Ensures the image property is correctly processed in copy(). Also avoid the usage of the spread operator to avoid issues with bundlers, see #22618.

[gkjohnson]

Ensures the image property is correctly processed in copy().

Just a quick note that

this.texture.image = { ...this.texture.image };

is the same result as

this.texture.image = { ...source.texture.image };

since this.texture.image === source.texture.image after source.texture.clone().

[mrdoob]

This suggestion by @gkjohnson is nicer:

@Mugen87 suggestion looks good but Object.assign( {}, this.texture.image ) would work, as well

[Mugen87]

@gkjohnson Indeed, I've just misread the code. I've used source in the new version for better clarity.

@mrdoob Updated!

[mrdoob]

Thanks!