Limiting screenshots to the visible window

[hadiasghari]

The method CommandSequenec.save_screenshot() currently saves a screen shot of the FULL HTML page. This is not always desirable, and a parameter to limit the screenshot to the visible window is welcome.

The following screenshot from YouTube illustrates the point:

Two reasons for the parameter:

• For purposes such as identifying cookie-notices, only the visible browser window matters, and the difference in disk-use can be significant.
• Additionally, the full page rendering can cause Selenium timeouts, with frequent errors: "TaskManager ... Timeout while executing command, SAVE_SCREENSHOT, killing browser manager"

[englehardt]

Viewport-only screenshots are now the default in Selenium 3, which we moved to in #152. Thus CommandSequence::save_screenshot will only save the visible portion of the current page.

#156 adds a separate command for full page screenshots.