Screenshots does not work with non-session scoped browsers

[pelme]

When using --splinter-session-scoped-browser=false, the failure screenshot does not work. The browser gets shut down before the screenshot takes place.

There is no explicit dependency on the browser instance in browser_screenshot. It just looks at the requested fixtures and gets the browser. However, the browser instance may already have been shut down at the time when the screenshot capture happens.

I have a PR forthcoming.