i965/vs: Set brw_vs_prog_key::clamp_vertex_color to 0 when irrelevant.

Vertex color clamping is only relevant if the shader writes to
the built-in gl_[Secondary]{Front,Back}Color varyings.  Otherwise,
brw_vs_prog_key::clamp_vertex_color is never used, so we can simply
leave it set to 0.

This enables us to correctly predict the clamp_vertex_color key value
in the precompile for shaders which don't use those varyings.

Eliminates virtually all VS recompiles in Serious Sam 3's intro.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

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

@@ -431,8 +431,11 @@ static void brw_upload_vs_prog(struct brw_context *brw)
                            ctx->Polygon.BackMode != GL_FILL);
    }
 
-   /* _NEW_LIGHT | _NEW_BUFFERS */
-   key.clamp_vertex_color = ctx->Light._ClampVertexColor;
+   if (prog->OutputsWritten & (VARYING_BIT_COL0 | VARYING_BIT_COL1 |
+                               VARYING_BIT_BFC0 | VARYING_BIT_BFC1)) {
+      /* _NEW_LIGHT | _NEW_BUFFERS */
+      key.clamp_vertex_color = ctx->Light._ClampVertexColor;
+   }
 
    /* _NEW_POINT */
    if (brw->gen < 6 && ctx->Point.PointSprite) {
@@ -541,7 +544,9 @@ brw_vs_precompile(struct gl_context *ctx,
    memset(&key, 0, sizeof(key));
 
    brw_vec4_setup_prog_key_for_precompile(ctx, &key.base, bvp->id, &vp->Base);
-   key.clamp_vertex_color = ctx->API == API_OPENGL_COMPAT;
+   key.clamp_vertex_color =
+      (prog->OutputsWritten & (VARYING_BIT_COL0 | VARYING_BIT_COL1 |
+                               VARYING_BIT_BFC0 | VARYING_BIT_BFC1));
 
    success = do_vs_prog(brw, shader_prog, bvp, &key);