incremental decoding

[pumbur]

approach alternative to #39 and #40, but on the same path to #35 and #4.
somewhat inspired by libjpeg.

idea is to add SPNG_IO_SUSPENDED error and return it when not enough data available.
and read_fn should return either negative io errors or num of bytes read.

no additional callbacks and functions needed, when new data available user will call previously failed spng_decode_scanline again, which will ask read_fn callback for new data.