i965g: handle special vs outputs specially

i965g: handle special vs outputs specially

Where vs output semantic tags indicate an output is signficant for
fixed function processing (such as clipping, unfilled modes, etc),
retain information about that output so that we can get to it easily
later on.

Fix up the unfilled processing, but hard-wire edgeflag to one for now.

With this change, trivial/tri-unfilled works.