Add multimon support to GHOST_DisplayManagerWin32.cpp (It seemed to work before, but recent changes to GHOST_ASSERT caused it to abort if WITH_ASSERT_ABORT was configured).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53923 954f8c5b-7b00-dc11-b283-0030488c597c