This is only meaningful for indirect contexts, and all it does is
(maybe) prevent a flush when switching away from an indirect context.
Indirect contexts aren't worth optimizing for, and Mesa tracks whether
a flush is needed anyway.

Careful readers will note that ReadPixels would reset the flag even
though it doesn't imply a flush!

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>