RenderTarget: Preserve multiview in copy()

[Kundan-CR7]

Fixed #33284

Description

The RenderTarget.copy() method does not copy the multiview property from the source instance.

The constructor initializes multiview using options.multiview, but it was missing in the copy() method. This leads to inconsistent behavior when cloning render targets configured with multiview enabled.

This PR fixes the issue by copying the multiview property:

this.multiview = source.multiview;

This ensures that cloned render targets retain the same configuration as the source.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	360.01 85.46	360.01 85.46	+0 B +0 B
WebGPU	633.38 175.63	633.38 175.63	+0 B +0 B
WebGPU Nodes	631.5 175.33	631.5 175.33	+0 B +0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	492.21 120.03	492.24 120.04	+27 B +4 B
WebGPU	705.41 190.51	705.43 190.51	+27 B +6 B
WebGPU Nodes	654.63 177.77	654.65 177.77	+27 B +7 B

[Kundan-CR7]

@Mugen87 please review this PR