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

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.22 79.04	339.4 79.07	+185 B +35 B
WebGPU	476.91 132.18	476.91 132.18	+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	464.86 112.03	+185 B +36 B
WebGPU	545.94 147.85	545.94 147.85	+0 B +0 B
WebGPU Nodes	501.82 137.55	501.82 137.55	+0 B +0 B