add dual-buffering for rtmp

[jeroen]

This is a small tweak to get less buffering issues: after a buffer has been initially filled, the bufferlength is set to e.g. 4x the buffersize, so RTMP streams won't stall if a short bandwidth glitch occurs.

When the buffer percentage goes below e.g. 20% or when a seek/pause/stop is performed, the initial, short bufferlength is set again.

This is a nice, simple fix to make RTMP a little more forgiving on bandwidth glitches.

[jeroen]

The bufferlength on RTMP is now quadrupled every time the buffer is filled. If it empties (bad connection or a seek), it's reset to the short state again.