This will let us use a subset of the full texture, which can
be necessary in the case that converters put padding around
content in dmabufs. The naming follows the Wayland viewporter
spec.
For now, make all callers pass the full texture rect.
We are going to introduce another rect, so better to be clear in
naming. We are following the naming of the Wayland viewporter spec
and call the rectangle that we drawing into the dest(ination).
Instead, do it all in attach(), which becomes more and more like
ConfigureWindow. This is good, because it will let us take the
above-ness into account when making decisions about attaching.
Add api to allow creating subsurfaces, attaching textures to them,
and changing the stacking order.
This is just the api, there is no implementation yet.
