Parallel calls to Page.screenshot interfere with each other

[aslushnikov]

Consider the following example, which takes 10 screenshots, each of a different 50pxx50px square.

browser.newPage().then(async page => {
    await page.navigate('http://example.com');
    var promises = [];
    for (var i = 0; i < 10; ++i) {
        promises.push(page.screenshot({
            path: i + '.png',
            clip: {x: 50 * i, y: 0, width: 50, height: 50},
        });
    }
    await Promise.all(promises);
    browser.close();
});

Unfortunately, this doesn't work - calls to page.screenshot interfere with each other, failing to clip proper rect.