Fix rasterize example exit status on failure.

https://github.com/ariya/phantomjs/issues/11911
Use Array.prototype.slice in window.callPhantom.

window.callPhantom formerly used Array.prototype.splice for cloning
its arguments to the internal _phantom.call. This relied on
non-standard behaviour of the splice method when only a single
argument was passed to it (it requires 2 arguments).

The correct method for cloning the arguments array is to use
Array.prototype.slice, which accepts a single argument (index) and
returns a new array from the specified index.

https://github.com/ariya/phantomjs/issues/12306