WebGPURenderer: Fix flipY and cache key when generateMipmap

[sunag]

Related issue: #32906

Description

Fix flipY and cache key when generateMipmap and update function signature.

This should fix the webgpu_performance example.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	359.21 85.29	359.21 85.29	+0 B +0 B
WebGPU	623.73 173.39	623.7 173.38	-24 B -4 B
WebGPU Nodes	622.31 173.13	622.28 173.13	-24 B -3 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	490.47 119.49	490.47 119.49	+0 B +0 B
WebGPU	690.39 186.56	690.36 186.56	-24 B -3 B
WebGPU Nodes	639.77 173.97	639.75 173.96	-24 B -4 B

[greggman]

😅