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

Adds two tests verifying that custom shaders can correctly sample
both color and depth framebuffer textures via setUniform(), which
is the pattern used by the Framebuffer Blur example.

- custom shader sampling only color texture
- custom shader sampling both color and depth textures

Both tests confirm the library sets uModelViewMatrix = model x view
correctly and that depth textures are readable via uniform sampler2D.

Refs: #8940

Author: vamshidevara

test/unit/webgl/p5.Framebuffer.js

@@ -669,6 +669,102 @@ suite('p5.Framebuffer', function() {
       });
   });
 
+  suite('custom shader with depth uniform', function() {
+    test('custom shader sampling only color texture', function() {
+      myp5.createCanvas(10, 10, myp5.WEBGL);
+
+      const fbo = myp5.createFramebuffer();
+      fbo.draw(() => {
+        myp5.background(255);
+        myp5.noStroke();
+        myp5.fill('red');
+        myp5.box(5, 5, 5);
+      });
+
+      const vertexShader = `
+        attribute vec3 aPosition;
+        attribute vec2 aTexCoord;
+        uniform mat4 uModelViewMatrix;
+        uniform mat4 uProjectionMatrix;
+        varying vec2 vTexCoord;
+        void main() {
+          vTexCoord = aTexCoord;
+          gl_Position = uProjectionMatrix * uModelViewMatrix * vec4(aPosition, 1.0);
+        }
+      `;
+      const fragmentShader = `
+        precision highp float;
+        varying vec2 vTexCoord;
+        uniform sampler2D img;
+        void main() {
+          gl_FragColor = texture2D(img, vTexCoord);
+        }
+      `;
+
+      const customShader = myp5.createShader(vertexShader, fragmentShader);
+
+      myp5.background(0);
+      myp5.noStroke();
+      myp5.shader(customShader);
+      customShader.setUniform('img', fbo.color);
+      myp5.plane(myp5.width, -myp5.height);
+
+      assert.deepEqual(myp5.get(5, 5), [255, 0, 0, 255]);
+    });
+
+    test('custom shader sampling both color and depth textures', function() {
+      myp5.createCanvas(10, 10, myp5.WEBGL);
+
+      const fbo = myp5.createFramebuffer();
+      fbo.draw(() => {
+        myp5.background(255);
+        myp5.noStroke();
+        myp5.fill('red');
+        myp5.box(5, 5, 5);
+      });
+
+      const vertexShader = `
+        attribute vec3 aPosition;
+        attribute vec2 aTexCoord;
+        uniform mat4 uModelViewMatrix;
+        uniform mat4 uProjectionMatrix;
+        varying vec2 vTexCoord;
+        void main() {
+          vTexCoord = aTexCoord;
+          gl_Position = uProjectionMatrix * uModelViewMatrix * vec4(aPosition, 1.0);
+        }
+      `;
+      const fragmentShader = `
+        precision highp float;
+        varying vec2 vTexCoord;
+        uniform sampler2D img;
+        uniform sampler2D depth;
+        void main() {
+          vec4 color = texture2D(img, vTexCoord);
+          float d = texture2D(depth, vTexCoord).r;
+          gl_FragColor = vec4(color.rgb * (1.0 - d), 1.0);
+        }
+      `;
+
+      const customShader = myp5.createShader(vertexShader, fragmentShader);
+
+      myp5.background(0);
+      myp5.noStroke();
+      myp5.shader(customShader);
+      customShader.setUniform('img', fbo.color);
+      customShader.setUniform('depth', fbo.depth);
+      myp5.plane(myp5.width, -myp5.height);
+
+      // The box is at z=0 with camera at z=800.
+      // Depth at center should be ~0.91 (232/255).
+      // Color should be dimmed: 255 * (1 - 0.91) ≈ 23
+      // Just check it's not full red (not 255) and not zero
+      const pixel = myp5.get(5, 5);
+      assert(pixel[0] < 200, 'red should be significantly dimmed by depth');
+      assert(pixel[0] > 0, 'red should still be visible');
+    });
+  });
+
   suite('saveCanvas', function() {
     test('should download a png file', async () => {
       myp5.createCanvas(100, 100, myp5.WEBGL);