RenderTarget: Fix depth texture assignment regression.

[Spiri0]

I fixed a small bug in RenderTarget.js. I corrected the usage of depthTexture assignment in the RenderTarget constructor. Now using the setter function again like in r174 and before. The setter ensure the correct _depthTexture assignment and proper renderTarget linkage.

Fixed #30895

A lot of changes were made in #30633, and this may have gotten lost

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.49 78.38	336.49 78.38	+0 B +0 B
WebGPU	542.05 150.13	542.05 150.13	+0 B +0 B
WebGPU Nodes	541.51 150.03	541.51 150.03	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.42 112.22	465.45 112.23	+23 B +4 B
WebGPU	614.89 166.17	614.91 166.18	+23 B +3 B
WebGPU Nodes	569.88 155.58	569.9 155.59	+23 B +4 B

[Mugen87]

Good catch!