i965/clip: Add support for gl_ClipVertex

When clipping against a user clip plane, and gl_ClipVertex is provided
in the vertex, use it instead of hpos.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965/vs: add support for emitting gl_ClipVertex

Removes the special-case suppression of gl_ClipVertex in the VUE map.

Also calculate vertex outcodes for user clip planes based on
gl_ClipVertex if written; otherwise gl_Position.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>