Fix trap- and triangle rendering to windows

[Søren Sandmann Pedersen]

The following patches to pixman and X server respectively should fix
trapezoid rendering to windows. Before drawable offsets were ignored.

Fixing this uncovered that the pixman API I added was pretty broken
because it would always treat the trapezoid coordinates as if they
were in destination space, which had the effect that changing
dst_x/dst_y only moved the source image around, and not the
trapezoids.

These patches fix that issue. It is a change to the semantics of the
pixman call, and if you apply either of these patches separately, all
trapzoid rendering will be garbled.

Hopefully, we can get these reviewed quickly so that I can make a
non-broken pixman release.


Thanks,
Soren