Increase tolerance to 8 from 3. (KhronosGroup#3216)

Found necessary on multiple platforms in http://crbug.com/1165751 .

Follow-up to KhronosGroup#3214 .

sdk/tests/conformance/glsl/bugs/vector-matrix-constructor-scalarization.html

@@ -149,12 +149,14 @@
     // Buffers returned from setupQuad above, and ignored, are already bound.
     wtu.drawUnitQuad(gl);
 
-    // Top and bottom halves should be equal. Go through one
+    // Top and bottom halves should be roughly equal. Go through one
     // horizontal scanline in the middle.
     const compareHeight = sz / 4;
     let pixelValue = new Uint8Array(4);
     let allEqual = true;
-    const tolerance = 3;
+    // Empirically found that tolerance between the top and bottom
+    // needs to be up to roughly 8 on some platforms.
+    const tolerance = 8;
     let tempBuf = new Uint8Array(4);
     // Step over some pixels to spew slightly fewer comparison messages.
     for (let x = 0; x < sz; x += 4) {