Avoid Intel Ultra 7 graphical glitches during vertex discard (#8730)

Co-authored-by: pmconne <[email protected]>

Author: markschlosseratbentley

common/changes/@itwin/core-frontend/workaround-intel-driver-ultra-7_2025-11-06-13-50.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@itwin/core-frontend",
+      "comment": "Work around Intel Ultra 7 vertex discard glitch",
+      "type": "none"
+    }
+  ],
+  "packageName": "@itwin/core-frontend"
+}

core/frontend/src/internal/render/webgl/ShaderBuilder.ts

@@ -819,14 +819,20 @@ export class VertexShaderBuilder extends ShaderBuilder {
       main.addline("  v_color = baseColor;");
     }
 
+    /*
+    There is currently a driver glitch which is known to affect much of the Intel Ultra 7 family of chipsets when using Intel driver from some point after driver version 32.0.101.6078 (9/13/2024).
+
+    The graphics driver will glitch when discarding triangles using the vertex shader (setting all vertices to the same value) if gl_Position was not initialized to a valid position beforehand.
+
+    The workaround for this bug involves ensuring that gl_Position is initialized to a valid position before attempting a discard using a degenerate triangle.
+    */
+    main.addline("  gl_Position = computePosition(rawPosition);");
     const checkForDiscard = this.get(VertexShaderComponent.CheckForDiscard);
     if (undefined !== checkForDiscard) {
       prelude.addFunction("bool checkForDiscard()", checkForDiscard);
       main.add(vertexDiscard);
     }
 
-    main.addline("  gl_Position = computePosition(rawPosition);");
-
     const finalizePos = this.get(VertexShaderComponent.FinalizePosition);
     if (undefined !== finalizePos) {
       prelude.addFunction("vec4 finalizePosition(vec4 pos)", finalizePos);