support window resize in gngr driver, use it to resize to 800x640

[hrj]

Currently, we are using maximised windows. But maximum size is overkill for most tests. It results in lots of blank areas, which waste computation during image comparison.

Something like 800x640 pixels should be fine for most of the tests.

[hrj]

I have implemented this, and see huge time savings. Will be committing soon.