nir: handle edge flags in nir_create_passthrough_gs

nir: handle edge flags in nir_create_passthrough_gs

`nir_create_passthrough_gs` will now take a boolean argument to decide
whether it needs to handle edgeflags.

When true is passed it will output a line strip where edges that
shouldn't be visible are not emitted.

This is usefull because geometry shaders will generally throw away
edgeflags so for a passthrough GS to act transparently it needs to emulate them.

Acked-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21238>