Fix: prevent uSampler uniform from being reset for user shaders

[Scoutersq]

Resolves #8200

Changes:

• Fixed a bug in WebGL shaders where the uSampler uniform was being reset to an empty texture, causing user-defined shaders to render blank.
• The fix ensures uSampler is only reset for internal p5.js shaders, not for shaders where the user manually set the uniform.
• Added a manual test file fix-shader.html to verify the fix: drawing a red rectangle using a custom FBO shader works correctly.

Screenshots of the change:

• Red rectangle appears correctly when using the fix-shader.html test (previously rendered blank).

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Manual test file (fix-shader.html) included
• Unit tests are included / updated

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Thank You!

[perminder-17]

I think the issue is in 2.x versions.

p5.js version

2.0.5 (2.0+)

We need to retarget it to dev-2.0 branch.

[davepagurek]

I'm going to close this since another contributor has asked first, as per our contribution guidelines.