gdi32/tests: Use virtual screen size instead of primary monitor screen size.

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=9028

Your paranoid android.


=== WNT4WSSP6 (32 bit clipping) ===
clipping.c:438: Test failed: expected 0,0-0,0, got 0,0-800,600