WebGLRenderer: Add support for copying mipmap data between textures

[gkjohnson]

Related issue: #29710 (comment)

Description

Adds support for copying data to and from mipmap levels using the copyTextureToTexture function.

• Adds a new srcLevel argument to the function
• Attempts to retain backwards compatibility with old signature
• Update docs
• Default to copying the appropriate mip map size
• Creates "scratch" frame buffers to reuse to avoid cases where other attachments are already bound but of different scales.

cc @CodyJasonBennett

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	691.65 171.38	692.66 171.56	+1.01 kB +174 B
WebGPU	822.12 221.9	822.12 221.9	+0 B +0 B
WebGPU Nodes	821.63 221.78	821.63 221.78	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	464 112.17	465.01 112.35	+1.01 kB +181 B
WebGPU	542.65 146.78	542.65 146.78	+0 B +0 B
WebGPU Nodes	498.65 136.58	498.65 136.58	+0 B +0 B