draw: fix vertex id computation

draw: fix vertex id computation

vertex id has to be unaffected by the start index (i.e. when calling
draw arrays with start_index = 5, the first vertex_id has to still
be 0, not 5) and it has to be equal to the index when performing
indexed rendering (in which case it has to be unaffected by the
index bias). This fixes our behavior.

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