The idea here is to eliminate the set_edgeflags() call in pipe_context
by treating edgeflags as a regular vertex element.

Edgeflags provoke special treatment in hardware, which means we need to
label them in some way, in this case we'll be passing them through the
vertex shader and labelling the vertex shader output with a new TGSI
semantic (TGSI_SEMANTIC_EDGEFLAG).