Take advantage of built in support for full page screenshots

[nextlevelbeard]

Is your feature request related to a problem? Please describe.
Often, the full page screenshots are unreliable and contain ghosting of elements due to scrolling effects.
For example: see #125

Describe the solution you'd like
Multiple browsers have built in support for this that deliver a clean and ghost free image.

For Firefox, we can create a client side script that calls the :screenshot command. See docs here:
https://firefox-source-docs.mozilla.org/devtools-user/taking_screenshots/index.html#taking-screenshots-with-the-web-console

Then execute it with the executor function in getFullPageScreenshotsDataDesktop:

visual-testing/packages/webdriver-image-comparison/src/methods/screenshots.ts

Line 352 in b40be9d

export async function getFullPageScreenshotsDataDesktop(

Describe alternatives you've considered

We could also call the webdriverIO API: browser.fullPageScreenshot(), if the browser is available to us.
https://webdriver.io/docs/api/gecko/#fullpagescreenshot

[christian-bromann]

We should update the saveScreenshot command in WebdriverIO core to use WebDriver Bidi primitives for taking screenshots. They provide full flexibility including full page screenshots.

[wswebcreation]

Thanks for your FR @nextlevelbeard , I've investigated the Firefox one in the past and wasn't fully reliable. But I can do a research again

@christian-bromann That would be great! Adding a flag for fullpage might be very useful