gtk: Fix mouse warping with gtk3

gtk: Fix mouse warping with gtk3

We were using the wrong coordinates, this fixes things to match the
original gtk2 implementation.

You can see this error in action by using -vga qxl, however even after this
patch the mouse warps in small increments up and to the left, -7x and -3y
pixels at a time, until the pointer is warped off the widget. I think it's
a qxl bug, but the next patch covers it up.

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>