opusenc: increase max payload size to 4000 bytes

1275 is the maximum size of a frame, but the encoder may return
up to 3 frames, and we need a few extra bytes for TOC, etc. We
use 4000, which is a bit more, and suggested in the libopus docs.
mfc: fix input dequeue for odroid

https://bugzilla.gnome.org/show_bug.cgi?id=721027
opusparse: Don't send caps in GstBaseParse::start()

This is too early and will confuse the event order. The other
code that sets the caps is at the right position and does it
properly already.
opencv: The plugin works with 2.4.7 too

Reported by kwm on IRC.