[winex11] wintab.c: use atan2 instead of atan

[Dan Kegel]

Friends don't let friends call atan.

This fixes a crash reported in OpenCanvas 1.1
http://bugs.winehq.org/show_bug.cgi?id=11838
-------------- next part --------------
A non-text attachment was scrubbed...
Name: atan2.patch
Type: text/x-patch
Size: 1012 bytes
Desc: not available
Url : http://www.winehq.org/pipermail/wine-patches/attachments/20080306/256ac177/attachment.bin