i965: Advertise our vertex shader texture units.

Previously, we advertised 0 VS texture units.  Now that we have proper
support for using the sampling engine in the VS, we can advertise 16,
which is conveniently the number required for OpenGL 3.0.

v2: Enable on Gen4.  I hacked up my tests to not use flat ivec varyings
    and they pass.

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

src/mesa/drivers/dri/i965/brw_context.c

@@ -159,7 +159,7 @@ brwCreateContext(int api,
    ctx->Const.MaxTextureCoordUnits = 8; /* Mesa limit */
    ctx->Const.MaxTextureUnits = MIN2(ctx->Const.MaxTextureCoordUnits,
                                      ctx->Const.MaxTextureImageUnits);
-   ctx->Const.MaxVertexTextureImageUnits = 0; /* no vertex shader textures */
+   ctx->Const.MaxVertexTextureImageUnits = BRW_MAX_TEX_UNIT;
    ctx->Const.MaxCombinedTextureImageUnits =
       ctx->Const.MaxVertexTextureImageUnits +
       ctx->Const.MaxTextureImageUnits;