[rbp/omxplayer] Move the buffer of the decoded audio to output port

The buffers between host and gpu components are duplicated, so 2 seconds of 8 channel 48kHz audio requires 3MB.
That memory is required on both host and gpu. We want this buffering to provide protection from underrun when the arm is busy.

By moving the majority of the buffering from omx_decoder's input to output port, we save that memory on the arm.
Also the output port is 16-bit integer, rather than float, so we halve the size in addition.