output example: set the stream timebase

This is required by the new API.
Merge commit '884f7c975f0af25febe86660e87bf3b2165a0309'

* commit '884f7c975f0af25febe86660e87bf3b2165a0309':
  output example: set the stream timebase

Conflicts:
	doc/examples/muxing.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples/muxing: move swr context to OutputStream

Idea from: 56f98e340fca894a76d1ddbe33118b8d8c4db34a
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples/muxing: pick a supported channel layout if stereo isnt supported by the encoder

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples/muxing: select a supported sample rate for the encoder, favor 44100

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples/muxing: Support setting flags, for example for setting bitexact

This would allow the example to be used in regression tests

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>