Close the image segmenter result.

This doesn't currently do anything useful, since we're running
tflite on the CPU, but might avoid bugs if we get GPU support.

static/background-blur-worker.js

@@ -47,6 +47,7 @@ onmessage = e => {
                 let id = new ImageData(width, height);
                 for(let i = 0; i < mask.length; i++)
                     id.data[4 * i + 3] = mask[i];
+                result.close();
                 createImageBitmap(id).then(ib => {
                     postMessage({
                         bitmap: bitmap,