i965: Expose gl_BaseVertex via a vertex attribute.

i965: Expose gl_BaseVertex via a vertex attribute.

Now that we have the data available, we need to expose it to the
shaders.  We can reuse the same vertex element that we use for
gl_VertexID, but we need to back it by an actual vertex buffer.

A hardware restriction requires that vertex attributes coming from a
buffer (STORE_SRC) must come before any other types (i.e. STORE_0).
So, we have to make gl_BaseVertex be the .x component of the vertex
attribute.  This means moving gl_VertexID to a different component.

I chose to move gl_VertexID and gl_InstanceID to the .z and .w
components, respectively, to make room for gl_BaseInstance in the .y
component (which would also come from a buffer, and therefore be
STORE_SRC).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>