WebGLRenderer: Align transmission render target samples with canvas.

[mrdoob]

Description

This PR ensures the transmission render target uses the same antialiasing level as the main canvas output, rather than always using 4 samples regardless of the renderer's antialias setting.

(Made using Claude Code with Opus 4.5)

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.39 83.06	350.43 83.06	+43 B +6 B
WebGPU	614.38 170.61	614.38 170.61	+0 B +0 B
WebGPU Nodes	612.99 170.35	612.99 170.35	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.43 117.86	482.47 117.86	+43 B +6 B
WebGPU	685.72 186.38	685.72 186.38	+0 B +0 B
WebGPU Nodes	635.56 173.57	635.56 173.57	+0 B +0 B