GLSLNodeBuilder: Fix usage of flat qualifier.

[Mugen87]

Related issue: -

Description

When testing webgpu_xr_cubes on an Android device with Chrome, I have noticed a runtime error in the browser console. A vertex shader does not compile correctly since the flat interpolation qualifier is used without a varying which is not valid. Meaning:

flat /*out*/ uint cameraIndex;

To fix the issue, GLSLNodeBuilder now only uses flat in combination with out.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.27 78.29	336.27 78.29	+0 B +0 B
WebGPU	517.04 143.54	517.06 143.53	+18 B -7 B
WebGPU Nodes	516.51 143.43	516.52 143.42	+18 B -8 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.25 112.13	465.25 112.13	+0 B +0 B
WebGPU	589.05 159.7	589.07 159.69	+18 B -8 B
WebGPU Nodes	544.43 149.26	544.45 149.25	+18 B -8 B