draw/so: uses the correct index with the pre clipped coordinates

draw/so: uses the correct index with the pre clipped coordinates

pre_clip_pos is a float[4] we just used (*float)[4] to be able to
jump within the array of vertex_headers with it. So if the idx
happened to be anything but 0, we'd actually read from some garbage
in memory. Change it to just be a simple pointer instead of casting
it to something that it's not. As suggested by Jose.

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