draw: reset the vertex id when injecting new primitive id

draw: reset the vertex id when injecting new primitive id

Without reseting the vertex id, with primitives where the same
vertex is used with different primitives (e.g. tri/lines strips)
our vbuf module won't re-emit those vertices with the changed
primitive id. So lets reset the vertex id whenever injecting
new primitive id to make sure that the vertex data is correctly
emitted.

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