apedec: do not buffer decoded samples over AVPackets

Only consume an AVPacket when all the samples have been read.

When the rate of samples output is limited (by the default value
of max_samples), consuming the first packet immediately will cause
timing problems:

- The first packet with PTS 0 will output 4608 samples and be
consumed entirely
- The second packet with PTS 64 will output the remaining samples
(typically, a lot, that's why max_samples exist) until the decoded
samples of the first packet have been exhausted, at which point the
samples of the second packet will be decoded and output when
av_decode_frame is called with the next packet).

That means there's a PTS jump since the first packet is 'decoded'
immediately, which can be seen with avplay or mplayer: the timing
jumps immediately to 6.2s (which is the size of a packet).

Sample: http://streams.videolan.org/issues/6348/Goldwave-MAClib.ape
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>