playbin: Keep a reference to the playsink sinkpads

Otherwise playsink might get shut down without us noticing
that our pad references are gone now.

Probably fixes https://bugzilla.gnome.org/show_bug.cgi?id=733165
basetextoverlay: always intersect with the filter caps

Avoids returning values that upstream can't produce

https://bugzilla.gnome.org/show_bug.cgi?id=733916
basetextoverlay: rework caps negotiation

Make textoverlay negotiate caps more correctly.

1) Check what caps we received in the video-sink
2) If it already has the overlay meta -> use it directly
3) If it doesn't, textoverlay try adding the overlay meta and using it,
   if downstream doesn't support it, just use what is received in the
   video-sink
4) Check if the allocation query also supports the meta to enable
   really using it

Before it wasn't really doing renegotiation of any kind, just
re-checking if it should use the overlay meta or not

Also had to update the caps in the test as memory:SystemMemory seems
to be required when you use a caps feature otherwise intersection/subset
checks will fail.

https://bugzilla.gnome.org/show_bug.cgi?id=733916
playsinkconvertbin: Make sure to intersect raw caps with our converter caps

Otherwise we end up allowing video/x-raw with arbitrary caps features that are
not handled by our converters.

https://bugzilla.gnome.org/show_bug.cgi?id=734683
basetextoverlay: fix caps negotiation filter