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
WIP: add GS visitors