draw: Allocate the output buffer for output primitives

draw: Allocate the output buffer for output primitives

We were allocating the output buffer but using the input
primitives. We need to allocate that buffer using the
maximum number of output, not input, primitives.

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