Before this, the mp3 file would be read into the in-memory buffer
only when new samples were actually needed. This meant that the time
to read mp3 content always counted against the ~22ms audio buffer length.
Now, when there's at least 1 full disk block of free space in the input
buffer, we can request that the buffer be filled _after_ returning from
audiomp3_mp3file_get_buffer and actually filling the DMA pointers. In
this way, the time taken for reading MP3 data from flash/SD is less
likely to cause an underrun of audio DMA.
The existing calls to fill the inbuf remain, but in most cases during
streaming these become no-ops because the buffer will be over half full.
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