[Cin] New feature: enhanced Motion plugin

[Andrea paz]

Trying to run the sge test, I loaded tm1.xml. I opened the motion
plugin window and realized I have "Play every frame" unchecked. L'ho
attivato. I closed the plugin window (with the warning message in
red); I reopened it (message in green) and after a couple of seconds I
had a crash with dump. I did it all again as root and got a crash with
dump again, which I report in attachment. To be precise the crash
happens when I move the plugin window to another position. If I don't
do the checked/unchecked steps of "Play every frame" I don't crash, if
I do I crash.
At the terminal there are the following lines, but I always have them,
not only in this case:

unjoined tids / owner 3
  00007f46e9502640 / 00007f484b7fe640 12RenderEngine
  00007f4708540640 / 00007f484b7fe640 12RenderEngine
  00007f46fb526640 / 00007f47f5ffb640 12RenderEngine

On the occasion of the crash I have:

** segv at 0x556921190bf4 in pid 179611, tid 179707
writing debug data to /tmp/cinelerra_179611.dmp
lock_items: 22
lock_frees: 7
** dump complete