fdo#70393: move Mesa to a subdir of external

Change-Id: Ib3c0516297b0bd68b59c8cfc5c5488005c059c27
Reviewed-on: https://gerrit.libreoffice.org/6557
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>