ViewHelper: Adjust viewport position for WebGPU renderer

[Child-qjj]

Closes: #30729

Description
set correct y-axis position when renderer.isWebGPURenderer is true

const x = domElement.offsetWidth - dim;
const y = renderer.isWebGPURenderer ? domElement.offsetHeight - dim : 0;

renderer.clearDepth();

renderer.getViewport( viewport );
renderer.setViewport( x, y, dim, dim );