draw: fix clipper invocation statistics

draw: fix clipper invocation statistics

We need to figure out the number of invocations of the clipper
before the emit, because in the emit we are after clipping
where the number of primitives will be equal to number of clipper
invocations minus the clipped primitives. So our computations
were always off by the number of clipped primitives.

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