(Preliminary) sample playback cursor

[prayerie]

(maybe the way I have done things here isn't really optimal, hence the draft status)

Adds required prerequisites to libntxm for sample cursor display, specifically:

the struct SampleCursor which contains information needed by the sample display to render the cursor: specifically the calculated current position of played audio (playbackpos), the frequency it is currently playing at (playbackfreq) to calculate how far the cursor should move, whether the note is playing, whether it is currently playing backwards (in the case of a ping-pong loop), and finally the sample and instrument idx so the sample display can ignore it if it's not relevant to the selected sample

in order to accurately calculate the cursor's position for finestep commands bendNote and bendNoteDirect also now return the frequency lookup result (or NULL), which is propagated through Instrument::bendNote[Direct] (it seemed like the simplest solution to me as player.cpp doesn't have linear_freq_table_lookup, and the sample's existing finetune property does not relate to effect commands.

also a new FIFO command to communicate the shared memory address of the array of SampleCursors

Also see: NitrousTracker/nitroustracker#205