Use the input data ring buffer to count input frames read

[padenot]

This is what we need. When starting a stream, there are two cases:

• There are already frames buffered in the ring buffer: we want to trim the input data to have just enough to not buffer
• There are no (or not enough) frames in the ring buffer: we want to provide silence to the callback

get_linear_input_data() underruns gracefully and fill the remaining space with silence.

[padenot]

@ChunMinChang, this is similar to what we do in the mac backend, conceptually, but there is a way to know how much is buffered there, to not trim when there is not enough data, instead of trimming in all cases and returning early when there's nothing to do.

[padenot]

If input_frame_count > nframes > _frames in input_buffermanager, then we may have a out-of-sync problem between input_frame_count and input_buffer_manager. I'd prefer we totally remove the input_frame_count and only use input_buffer_manager to calculate frames, if this is the case (IIRC, this is what macos backend does).

I'll do this, it's better.