When scaling part of an image we need to specify the source
coordinates in transformed coordinates. For large magnifications this
means we will get pretty large values.

Now, if e.g. src_x * transform is larger than 32765, then the
coordinate ends up outside the pixman 16bit image size, so the
rendering will not work.

The fix is to make the src_x/y offset part of the transformation.
This means its automatically transformed by the correct scaling, and
the coordinates passed into pixman are not (typically) over 16bit.

This reverts commit e13be77f33609cb3fdae354ce1f2686ae865f9e0.

This will prevent: 1) rendering problems (commands sent to the client in the wrong order)
2) sending commands for surfaces that haven't been created yet on the client side.

This used to be a callback for the vdi_port "data ready" interrupt,
which did indicate either data ready to read or data ready to write, but
this is no longer the case now that virtio-serial is used.

This seemingly simple fix prevents a race that needs to be fixed with
another patch, see freedesktop bz #29903