WebGPURenderer: Add ReadbackBuffer as parameter

[sunag]

Fixes #33281

Description

Add ReadbackBuffer as a parameter; this should help share the GPU buffer between attributes.

const readbackBuffer = new THREE.ReadbackBuffer( size );

const arrayBuffer = await renderer.getArrayBufferAsync( attribute, readbackBuffer );

/cc @gkjohnson

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	360.57 85.58	360.57 85.58	+0 B +0 B
WebGPU	635.53 176.41	635.58 176.46	+45 B +44 B
WebGPU Nodes	633.65 176.12	633.69 176.16	+45 B +43 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	492.95 120.2	492.95 120.2	+0 B +0 B
WebGPU	707.71 191.31	707.76 191.35	+45 B +40 B
WebGPU Nodes	656.93 178.57	656.98 178.61	+45 B +42 B