wlt: toolkit: fix sending two surface.attach() reqs in one frame

wlt: toolkit: fix sending two surface.attach() reqs in one frame

The need_frame and idle_frame logic was slightly wrong. We ended up not
correctly requesting the frame-cb after a _real_ frame-cb. Hence, the
idle-cb did simply send the next attach() request.

This fixes this small race and adds a comment how all this is supposed to
work.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>