process images parallel while interacting with the browser

[zinserjan]

Little refactoring to process images parallel to browser calls. In my test case it was up to 36% faster, but that depends on the hardware, number of images to stitch together, browser, etc...

Stats:

• url: https://github.com/zinserjan/wdio-screenshot
• method: saveDocumentScreenshot
• Measured before and after makeAreaScreenshot with console.time

	Before	After	Improvement
iPad Air 2 iOS 10.2 landscape	10825ms	6888ms	36%
Chrome v56 1024x768	6328ms	5184ms	18%
Firefox v51	1890ms	1485ms	21%

Note: Firefox performs best here, cause it emits non-retina images and doesn't need the normalization -> less image processing.