glimagesink: Only cache pool, don't manage it

glimagesink: Only cache pool, don't manage it

GLImage does not use any kind of internal pool. There was some
remaining code and comment stating that it was managing the
pool, and it was in fact setting the active state when doing
to ready state.

* Only create the pool if requested and in propose_allocation
* Cache the pool to avoid reallocation on spurious reconfigure
* Don't try to deactivate the pool (we don't own it)

https://bugzilla.gnome.org/show_bug.cgi?id=745705