*.fur files saved with newest unstable Furnace build yields an error

[The-Beesh-Spweesh]

Error: decoding test1/brunotime.fur

Caused by:
    0: parsing subsongs from test1/brunotime.fur
    1: SNG2 speed step multiplier must be non-zero

The newest unstable builds of Furnace have deprecated the speed step multiplier, which was a complication that only existed for DefleMask backwards compatibility, and that byte in the subsong chunk now has a new use (which isn't currently implemented in Furnace yet).