File start buffer is queued by prime().
File start buffer is queued by prime().

- Added _readFileStart flag to track whether the file start buffer
  has been filled yet.
- prime() will queue buffer 0 to read the file start if the flag is false.
- queue_buffer_for_read() sets the buffer's startFrame and frameCount
  fields based on already queued samples.
- enqueue_full_buffer() sets the _readFileStart flag if it is passed
  buffer 0.

flit/modular,flit/modular,flit/modular,flit/modular,flit/modular