Adds transcoding of multichannel audio to AC3 for the CoreAudioEngine.

Changes CoreAudioAE::MakeStream to detect if transcoding is possible and needed. The
transcoding state is passed to OpenCoreAudio where is plays a part in
the setup of the object. Mostly, transcode behaves like passthrough
here.

The actual transcoding happens in CoreAudioAEStream, were an
AEEncoderFFmpeg is instantiated and used if transcosing is called for.

The trickiest bit is the addition of a buffer for incoming audio that
needs to be encoded. The encoder needs to work in specific block sizes,
and the incoming stream is not always handed to us in appropriate chunks
so we need to buffer it up so we can process it. This required a little
fiddling with the delays to make sure sync was kept tight.
fixed bug with arial.ttf
[rbp/omxplayer] Avoid too many calls to GPU

We currently read the media time once per packet received from demuxer to determine gpu underrun.
We've found that TrueHD audio in particular produces ~1000 packets per second (whether it is the active track or not).
The cost of reading media time (from gpu) is high enough that 1000 calls per second makes us fail to keep up.
So, cache the media time, and only read it at most 50 times per second.
[rbp/omxplayer] Avoid audio codec when in passthrough modes

There is no need to open the ffmpeg audio codec when in passthrough mode.
Also there is no need to close/open the OMXAudio component when parameters (like number of channels) change in passthough.
This avoids an unpleasant stutter
Merge pull request #3203 from taxigps/fonts

fixed bug with arial.ttf
ActiveAESink: Fix GenarateNoise()
[osx] - fix warning about deprecated conversion between char * and string constant
[airplay] - fix possible deadlock when stopping picture airplay - thx @ulion
Merge pull request #3074 from ecopoesis/coreaudio-transcode-ac3

transcoding to AC3 for CoreAudio
Merge pull request #3207 from fritsch/active-ae-slidely-louder

ActiveAE: Fix double to float conversion in Noise Function
Merge pull request #3211 from popcornmix/gpu_calls

[rbp/omxplayer] Avoid too many calls to GPU
Merge pull request #3212 from popcornmix/avoid_audio_codec

[rbp/omxplayer] Avoid audio codec when in passthrough modes