Fix the audio mixer so that the samples are mixed at the right time.  The current method causes audio artifacts with multiple sources and short durations.
Merge branch 'master' of github.com:jgh-/VideoCore into audioMixerFix
fix build
New mixing method seems to work
If the time difference between the end of the previous sample and the start of the current sample < the duration of one mixer frame, paste it to the back of the previous sample.
Fix conflict
Merge pull request #77 from jgh-/audioMixerFix

Audio mixer fix