Changed WindowsViewer format to PNG

[radarhere]

Resolves #3695

[radarhere]

Here is a screenshot from AppVeyor.

[hugovk]

How do you get a screenshot from AppVeyor?

[radarhere]

I used ImageGrab to get the screenshot, saved it to file, then used https://transfer.sh/ to extract it. I imagine you could also use AppVeyor artifacts to similarly get the file out.

[nulano]

Another option, which I would have used, is remote desktop:

Pillow/.appveyor.yml

Lines 121 to 125 in 79a9981

	# Uncomment the following lines to get RDP access after the build/test and block for
	# up to the timeout limit (~1hr)
	#
	#on_finish:
	#- ps: $blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))