fix: only skip uSampler override when user explicitly set it

Replace isUserFillShader check with _userSetSampler flag on Shader,
tracking only explicit user calls to setUniform('uSampler', ...).
Guarded by _settingFillUniforms on renderer to distinguish internal
_setFillUniforms calls from user calls.

Fixes the texture() does not remain bound test which uses
baseMaterialShader().modify({}) with shader() — previously caught
by isUserFillShader check, preventing uSampler reset to empty texture
after pop().

Author: BHARATH0153

src/core/p5.Renderer3D.js

@@ -1496,17 +1496,11 @@ export class Renderer3D extends Renderer {
     fillShader.setUniform("uUseVertexColor", this._useVertexColor);
     fillShader.setUniform("uMaterialColor", this.states.curFillColor);
     fillShader.setUniform("isTexture", !!this.states._tex);
-    // We need to explicitly set uSampler back to an empty texture here.
-    // In general, we record the last set texture so we can re-apply it
-    // 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.
-    const isUserFillShader =
-      fillShader === this.states.userFillShader ||
-      fillShader === this.states.userImageShader;
-    if (this.states._tex || !isUserFillShader) {
+    this._settingFillUniforms = true;
+    if (this.states._tex || !fillShader._userSetSampler) {
       fillShader.setUniform("uSampler", this.states._tex || empty);
     }
+    this._settingFillUniforms = false;
     fillShader.setUniform(
       "uTint",
       this.states.tint?._getRGBA([255, 255, 255, 255]) ?? [255, 255, 255, 255]

src/webgl/p5.Shader.js

@@ -1103,6 +1103,13 @@ class Shader {
       return;
     }
 
+    // Track when the user explicitly sets uSampler so that
+    // _setFillUniforms knows not to override it with the empty texture
+    // when no texture is active via the p5 texture() API.
+    if (uniformName === 'uSampler' && !this._renderer._settingFillUniforms) {
+      this._userSetSampler = true;
+    }
+
     // In p5.strands-related code, where some of the code may be in
     // p5.webgpu.js instead of the main p5.js build, we generally use
     // duck typing instead of instanceof to avoid accidentally importing