draw: remove clip_vertex from vertex header

draw: remove clip_vertex from vertex header

vertex header had both clip_pos and clip_vertex.
We only really need one (clip_pos) because the draw llvm shader would
overwrite the position output from the vs with the viewport transformed.
However, we don't really need the second one, which was only really used
for gl_ClipVertex - if the shader didn't have that the values were just
duplicated to both clip_pos and clip_vertex. So, just use this from the vs
output instead when we actually need it.
Also change clip debug to output both the data from clip_pos and the
clipVertex output (if available).
Makes some things more complex, some things less complex, but seems more
easy to understand what clipping actually does (and what values it uses
to do its magic).

Reviewed-by: Brian Paul <brianp@vmware.com
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>