Support big screenshot

[ryuran]

Environment (please complete the following information):

• webdriver-image-comparison version: 0.13.2

Describe the bug
Web driver Image fail on big images.

Given error :

invalid value (typically too big) for the size of the input (surface, pattern, etc.)

Automattic/node-canvas#1374

To Reproduce
Try with an image ~40000px long.

Expected behavior
webdriver-image-comparison should be able to manage big images (useful for full page screen shot).

Log

invalid value (typically too big) for the size of the input (surface, pattern, etc.)

Additional context
this comment advice to split the image to avoid this issue.

[wswebcreation]

Thanks for this issue, I'll take a look at it when I have some more time