rtpmux: Unref pad template caps after usage

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734473
navseek: Unref peer pad after usage

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734476
imagefreeze: Unref pad template caps after usage

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734475
videomixer: Unref allowed caps after usage

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734474
rtph263ppay: Unref pad template caps after use

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734435