i965: Move brw_vs_prog_data::outputs_written into VUE map.

Future patches will allow for there to be separate VUE maps when both
a geometry shader and a vertex shader are in use.  When this happens,
we will want to have correspondingly separate outputs_written
bitfields.  Moving outputs_written into the VUE map will make this
easy.

For consistency with the terminology used in the VUE map, the bitfield
is renamed to "slots_written" in the process.
WIP: i965: Create a pointer in brw_context to the geometry output VUE map.  TODO: is vue_maps substructure dumb?

Currently, the GPU pipeline has one active VUE map in effect at any
given time--the one representing the layout of vertex data coming from
the vertex shader.  However, when geometry shaders are added, they
will have their own independent VUE map.  Later pipeline stages (clip,
sf, fs) will need to consult the geometry shader VUE map if a geometry
shader is in use, and the vertex shader VUE map otherwise.

This patch adds a new field to brw_context, vue_maps.geom_out, which
points to whichever VUE map should be used by later pipeline stages.
It also adds a new state flag, BRW_NEW_GEOM_OUT_VUE_MAP, which is
signalled whenever this pointer changes.

Since we don't support geometry shaders yet, vue_maps.geom_out is
currently set only by the brw_vs_prog state atom.
WIP: add GS visitors