Fix: allow single-arg atan() outside strands; add visual test

[Abhayaj247]

Resolves #8092

• The single-argument atan(x) was incorrectly treated as GLSL-only in strands, causing a friendly error and returning undefined when called outside a shader. This mirrors the noise() override issue fixed in Fix noise() getting overridden; add tests #8036.
• The fix marks the 1-arg overload as a normal p5 function while keeping the 2-arg GLSL overload atan(y, x) strands-only.

Changes :

• src/webgl/ShaderGenerator.js

  • In builtInGLSLFunctions, set:
    • atan single-arg overload: isp5Function: true
    • atan two-arg overload: remains isp5Function: false

• test/unit/visual/cases/math.js

  • Add atan_outside_strands visual integration test that draws the value of atan(0.5) to ensure it evaluates and no friendly error is triggered.

• test/unit/spec.js

  • Include the new visual test suite entry visual/cases/math.

Screenshots of the change :

• Visual test creates baseline showing text like “atan(0.5) ≈ 0.464”; subsequent runs match the baseline.

PR Checklist

• npm run lint passes
• Inline reference is included / updated (function behavior unchanged; no reference edits needed)
• Unit tests are included / updated (added visual/integration test)