vnc-enc-tight: use thread local storage for palette

currently the color counting palette is allocated from heap, used and destroyed
for each single subrect. Use a static palette per thread for this purpose and
avoid the malloc and free for each update.

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1467280846-9674-1-git-send-email-pl@kamp.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>