test: add regression tests for custom shader with framebuffer depth texture

[vamshi567bh]

Closes #8947.

changes

Adds two regression tests covering the custom shader + framebuffer depth texture pattern used by the Framebuffer Blur example (see #8940). No library code changes are needed — the library already handles this correctly

1. custom shader sampling only color texture — verifies a custom vertex+fragment shader with uModelViewMatrix / uProjectionMatrix can sample fbo.color via setUniform and produce correct output
2. custom shader sampling both color and depth textures — verifies a custom shader can sample fbo.color and fbo.depth simultaneously, using depth values to modulate the output color

The proposed matrix swap in PR #8947 was incorrect. All three multiply methods (#mult4x4, #multNxN, #mult3x3) consistently perform right-multiplication. The uModelViewMatrix is correctly computed as model × view at p5.Renderer3D.js:1437. The Framebuffer Blur example vertex shader (uProjectionMatrix * uModelViewMatrix * position) is mathematically equivalent to uProjectionMatrix * uViewMatrix * uModelMatrix * position.

The uSampler override bug that could affect custom shaders was already fixed by commits f75f227 and 355a04b (Jun 2-3, 2026).

PR Checklist

• npm run lint passes
• [Inline reference] is included / updated
• [Unit tests] are included / updated

[vamshi567bh]

@davepagurek please review thanks!