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>
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