Delay decreasing reference count in frame-parallel decoding.

Delay decreasing reference count in frame-parallel decoding.

The current decoding scheme will decrease the reference count
of the output frame when finish decoding. Then the application
could copy the frame from the decoder buffer to application buffer.
In frame-parallel decoding, a decoded frame will not be outputted
until several frames later which depends on thread numbers. So
the decoded frame's reference count should be decreased only
after application finish copying the frame out. But due to the
limitation of vpx_codec_get_frame, decoder could not know when
application finish decoding. So use a index last_show_frame to
release the last output frame's reference count.

Change-Id: I403ee0d01148ac1182e5a2d87cf7dcc302b51e63