audiobuffersplit: New element that splits raw audio buffers into equal-sized buffers

audiobuffersplit: New element that splits raw audio buffers into equal-sized buffers

This is useful e.g. if audio buffers should be exactly the duration of a
video frame, or if a audio buffers should never be too large because of
latency constraints.

The element is taking a fractional buffer duration, to allow working
with e.g. 1001/30000 as output duration and it accumulates rounding
errors in the buffer durations and compensates for them by making some
buffers one sample larger than the others.

https://bugzilla.gnome.org/show_bug.cgi?id=774689