eglglessink: Don't set the EAGLContext of the main thread and don't reuse it either

... instead create a new context for every sink instance.

https://bugzilla.gnome.org/show_bug.cgi?id=720421
eglglessink: Properly clean up renderbuffers and EAGL context
adpcmenc: Properly set output format

Otherwise this will just error out if we only set caps on the srcpad.