Finally fixed dumb mistake.

Stupidly changed the #defines for blend types to be 0-indexed without
realizing they were redundantly defined in another spot. Basic motion
seems to be working normally now, suggesting that the code surgery
didn't break anything critical.

TODO: test other functions, rigid tapping, etc. to make sure there are
no gross errors
Hacks to show tangent blends, trying to isolate velocity glitch
Successful hack to show x-axis tangent blends
Small tweaks to show that it works with circular arcs too.

Still need to verify that acceleration isn't glitchy, but so far it
looks good.