Fix cpu-render.html coordinate space

This should ensure that CPU/GPU diffs are actually useful,
and not improperly shifted.

test/integration/cpu-render.html

@@ -52,10 +52,15 @@
                 return { id, drawable };
             }).filter(Boolean);
             const color = new Uint8ClampedArray(3);
-            for (let x = -239; x <= 240; x++) {
-                for (let y = -180; y< 180; y++) {
-                    render.constructor.sampleColor3b([x, y], drawBits, color);
-                    const offset = (((179-y) * 480) + 239 + x) * 4
+            const vec = [0, 0];
+            for (let x = 0; x < 480; x++) {
+                // leftmost pixel is -240, rightmost is 239
+                vec[0] = x - 240;
+                for (let y = 0; y < 360; y++) {
+                    // bottommost pixel is -179, topmost is 180
+                    vec[1] = 180 - y;
+                    render.constructor.sampleColor3b(vec, drawBits, color);
+                    const offset = ((y * 480) + x) * 4;
                     cpuImageData.data.set(color, offset);
                 }
             }