draw: make sure clipping works with injected outputs

draw: make sure clipping works with injected outputs

clipping would drop the extra outputs because it always
used the number of standard vertex shader outputs, without
geometry shader or extra outputs. The commit makes sure
that clipping with geometry shaders which have more outputs
than the current vertex shader and with extra outputs correctly
propagates the entire vertex.

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