win32: Fix placement at initial position

win32: Fix placement at initial position

Positioning windows at 0,0 post creation failed, because it
was mapped with CW_USEDFAULT, but private->x/y still said 0,
so moving it to 0,0 did nothing. We now always position the
window at the right place, even when not mapped, but we
create it at CW_USEDEFAULT initially and store that position
before moving it to the right place.

This fixes the window sizing test in testgtk and the inital
position for the gimp toolbar.