draw: fix vbuf caching of vertices with inject front face

draw: fix vbuf caching of vertices with inject front face

Caching in the vbuf module meant that once a vertex has been
emitted it was cached, but it's possible for a vertex at the
same location to be emitted again, but this time with a different
front-face semantic. Caching was causing the first version of the
vertex to be emitted, which resulted in the renderer getting
incorrect front-face attributes. By reseting the vertex_id (which
is used for caching) we make sure that once a front-face info
has been injected the vertex will endup getting emitted.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>