When using Xpresent, the compositor maintains multiple pixmap buffers
and therefore needs to accumulate damages for those multiple buffers.

However, when using GLX or no vblank method at all, there is only one
buffer pixmap and there is no need to accumulate damage between buffers.

Currently, the compositor would accumulate damage indefinitely when
using GLX or no vblank, hence damaging way more than actually necessary.

Fix this by accumulating damages only when there is more than one
buffer.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>