i965/Gen4-5: allocate VUE slots for clipdistance if user clipping is enabled

Also adjust the SF URB read offset to account for there being two
additional slots of stuff it doesn't care about.
pull legacy clip lowering out of emit loop

allow emit_clip_distances on pre-gen6

fix

fix more
stop ignoring CLIP_DIST0 and CLIP_DIST1 in clip shader
i965/Gen4-5: Set clip flags from clip distances
i965/clip: Track offset into the vertex for clipdistance
push dp4 into load_clip_distance
use clipdistances for userclip
remove spurious clipvertex param
add vue map debug
fix: add BRW_NEW_INTERPOLATION_MAP to state dump
i965/fs: Fix inconsistency in the VUE map.

The SF never gets to see the clip distances. If we didn't ignore them
here, we got confused about the correct offsets for any varyings laid
out after them. (user-defined varyings would break, but gl_FrontColor
etc worked properly).

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