added function and improved argument order consistency

[superbigio]

I send you a PR for the following:

1. I added
   size_t ltc_encoder_get_bufferoffset(LTCEncoder *e)
   to query the write offset of the internal buffer.
   I found this very useful in a variety of situations, especially when the size of the internal buffer is larger than exactly 1 frame of LTC. In my specific user case I need to change FPS in real-time so I have to allocate a buffer large enough to hold the lowest FPS allowed. However I can see how this could come handy also in situations where you want to speed-up or slow-down the timecode.
   It is true that you get the offset when you get the buffer anyway, but sometimes I found myself in need of the offset only.

2. I changed and renamed
   ltcsnd_sample_t *ltc_encoder_get_bufptr(LTCEncoder *e, int *size, int flush)
   to
   int ltc_encoder_get_bufferptr(LTCEncoder *e, ltcsnd_sample_t *buf, int flush)
   The argument order of the latter function is now consistent with get_buffer, copy_buffer, etc...
   Obviously I deprecated the former and exported it exactly like I did previously for other functions. So nothing should break.

3. Comments updated to refer to the new function names