Make initialisation consistent for Selenium and non-Selenium drivers

[xurizaemon]

Looking at #30 today I noticed an inconsistency in behaviour (I think).

https://github.com/integratedexperts/behat-screenshot/blob/master/src/IntegratedExperts/BehatScreenshotExtension/Context/ScreenshotContext.php#L73-L92

This checks instanceof Selenium2Driver and fires $driver->start() then does a resize. However, not all drivers that support screenshots are Selenium2Driver (eg DMore/chrome-mink-driver's ChromeDriver.

So ChromeDriver retains resized dimensions between runs (until Chrome exits), while Selenium is consistent. That's not a problem for me - but to implement initial size from behat.yml config in that function for drivers other than Selenium2, we'd need to have that function accommodate those other drivers too.

[AlexSkrypnyk]

Agree. The codebase does not do it well.

We should not only add a better check, but also have multiple different drivers setup in CI to make sure that this extension works with all of them correctly.