fix uSampler override: simplify with user-shader check

Apply review feedback: instead of tracking _userSetUniforms with _isInternalSetUniform flag, only override uSampler when shader is built-in or texture is active. Removes _userSetUniforms Set, _isInternalSetUniform flag, and per-frame clearing in _update().

Author: BHARATH0153

src/core/p5.Renderer3D.js

@@ -1501,7 +1501,12 @@ export class Renderer3D extends Renderer {
     // the next time a shader is used. However, the texture() function
     // works differently and is global p5 state. If the p5 state has
     // been cleared, we also need to clear the value in uSampler to match.
-    fillShader.setUniform("uSampler", this.states._tex || empty);
+    const isUserFillShader =
+      fillShader === this.states.userFillShader ||
+      fillShader === this.states.userImageShader;
+    if (this.states._tex || !isUserFillShader) {
+      fillShader.setUniform("uSampler", this.states._tex || empty);
+    }
     fillShader.setUniform(
       "uTint",
       this.states.tint?._getRGBA([255, 255, 255, 255]) ?? [255, 255, 255, 255]

test/unit/webgl/p5.RendererGL.js

@@ -208,6 +208,29 @@ suite('p5.RendererGL', function() {
       expect(uSampler.texture.isFramebufferTexture).toBeFalsy();
       myp5.pop();
     });
+
+    test('user-set uSampler on custom shader is not overridden', function() {
+      myp5.createCanvas(10, 10, myp5.WEBGL);
+
+      const myShader = myp5.createFilterShader(`precision highp float;
+uniform sampler2D uSampler;
+varying vec2 vTexCoord;
+void main() {
+  gl_FragColor = texture2D(uSampler, vTexCoord);
+}`);
+
+      const fbo = myp5.createFramebuffer();
+      fbo.draw(() => myp5.background('red'));
+
+      myp5.shader(myShader);
+      myp5.noStroke();
+      myShader.setUniform('uSampler', fbo);
+
+      myp5.plane(myp5.width, myp5.height);
+
+      const pixel = myp5.get(5, 5);
+      assert.deepEqual(pixel, [255, 0, 0, 255]);
+    });
   });
 
   suite('default stroke shader', function() {