mesa: Fold gallium's texture border stripping into a core Mesa option.

We wanted to reuse this in the Intel driver.

v2: Move the flag to ctx->Const

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/state_tracker/st_extensions.c

@@ -222,6 +222,8 @@ void st_init_limits(struct st_context *st)
       _mesa_override_glsl_version(st->ctx);
       c->UniformBooleanTrue = ~0;
    }
+
+   c->StripTextureBorder = GL_TRUE;
 }