rtpvp8depay: don't drop small packets that are valid

When the optional I header is present and 8 bytes (ie. M bit not set),
the sanity check on buffer size was wrong and could drop small
packets.
rtpvp8depay: don't fixate framerate to 0/1

The depayloader does not know the framerate, hence it should not
fixate it.
rtpv8depay: ignore reserved bit in payload descriptor

Draft 10 says this bit MUST be ignored by the receiver.
rtpvp8: VP8 as RTP encoding name.
vp8dec: Report decoder errors.
vp8enc: encode first frame with pts 0
vp8enc: Add 'bitrate' property which does the same as 'target-bitrate'
vp8enc: enable rate limit force keyframes events

Hard code limit to 500 ms between each generated key frame. This
should ideally be a property and set to a value derived from RTT.

This avoids generating bursts of keyframes when receiving many
PLIs/NACKs in close succession.
vp8dec: Make sure resolution is correct before allocating output frame.