[rbp/omxplayer] Support planar float format audio directly

The update to ffmpeg1.2 has produced a signifcant performance hit on the Pi when decoding multichannel audio without passthrough.
This is because (most) audio codecs now output planar floating point, rather than interleaved S16 samples.
OpenMax/GPU can handle interleaved S16 samples directly, whereas the floating point planar samples get converted with swr_convert.
This is a slow operation, and a quick profile showed it to be taking about 20% of CPU.

I've added support on GPU for a new floating point planar format in audio_decode component.
This PR makes this format the default format for sending audio to the GPU. The GPU now also handles the 6->8 channel padding which used to be required,
so the UpMix operation is no longer required, which may improve performance compares to pre-ffpeg1.2 version.

This PR requires updated GPU firmware. (384946 or later).