draw: fix problem in screen-space interpolation clip code

draw: fix problem in screen-space interpolation clip code

I don't see how this could have ever worked right.

The screen-space interpolation code uses the vertex->data[pos_attr]
position which contain window coords.  But window coords are only
computed for the unclipped vertices; the clipped vertices have
undefined window coords (see draw_cliptest_tmp.h).

Use the vertex clip coords instead which are always defined.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=55476
(piglit fbo-blit-stretch failure on softpipe)

Note: This is a candidate for the 9.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>