WebGPURenderer: Fix viewportCoordinate in WebGLBackend

[sunag]

Related issue: #29225

Description

Fix viewportCoordinate if used RenderTaget in WebGLBackend.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	685.2 kB 169.6 kB	685.2 kB 169.6 kB	+0 B +0 B
WebGPU	823 kB 220.8 kB	823 kB 220.9 kB	+84 B +32 B
WebGPU Nodes	822.5 kB 220.7 kB	822.6 kB 220.8 kB	+503 B +127 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462 kB 111.5 kB	462 kB 111.5 kB	+0 B +0 B
WebGPU	522.2 kB 140.8 kB	522.3 kB 140.8 kB	+84 B +30 B
WebGPU Nodes	478.9 kB 130.6 kB	478.9 kB 130.7 kB	-43.26 kB +29 B