WebGLRenderer: Allow for binding and rendering into a 2d render target mipmap

[gkjohnson]

Related issue: #29779

Description

Adjusts setRenderTarget to afford binding and rendering into a specific mipmap level of a 2d render target texture. A scratch frame buffer is used to avoid cases where there are other attachments on the cached framebuffer without mipmaps (ie a depth buffer) resulting in an error when drawing (which was happening with 3d render targets, as well).

• Depth buffers & rendering into depth textures, multiple render targets are not supported.
• Mipmaps are now no longer automatically generated when rendering into a mipmap layer.

cc @CodyJasonBennett

edit looks like this will need some different handling for multisample targets but I'm not sure what the best way is to handle that Fixed by not rebinding mipmap level == 0 MSRT textures

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.22 79.04	339.41 79.07	+192 B +37 B
WebGPU	476.9 132.19	476.9 132.19	+0 B +0 B
WebGPU Nodes	476.2 132.01	476.2 132.01	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	464.67 111.99	464.87 112.03	+192 B +38 B
WebGPU	545.93 147.86	545.93 147.86	+0 B +0 B
WebGPU Nodes	501.81 137.56	501.81 137.56	+0 B +0 B

[CodyJasonBennett]

I'm very sick right now, so I can't give this my full attention until the end of next week.

Just a quick glance: bullet points are reasonable and code looks nonintrusive so far.

If I want hardware filtering for depth and shadows, I'll add it myself. No need for that burden now.