user32: Use whole window rectangle for UpdateLayeredWindow (try2)

[Nikolay Sivov]

try2: dirty rectangle is in window coords, zero based, so offset to intersect.

This is to be applied after previous patch. Demo app mentioned in it shows that
we should overwrite entire window, not only client area. Offset for source
context are corrected and verified with demo too. With this patch overlay
window in Powerpoint 2007 will fill whole edit area and wrong offset
caused by nonclient size is fixed.

Another thing Wine differs in is that source context bitmap should be
exactly the same size as destination window rectangle. We currently
don't check this, but native doesn't copy anything in this case (more testing
needed).