softpipe: implement seamless cubemap support. (v1.1)

softpipe: implement seamless cubemap support. (v1.1)

This adds seamless sampling for cubemap boundaries if requested.

The corner case averaging is messy but seems like it should be spec
compliant.

The face direction stuff is also a bit messy, I've no idea if that could
or should be simpler, or even if all my directions are fully correct!

v1.1: update comments, drop unneeded seamless calls for nearest, fix
if statement layout.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>