[tx] write a vsindex before the first blend (if needed)

[cjchapman]

Prior to this change, tx was not writing vsindex in CFF2 output, even if it was needed. The result was char strings that would not rasterize correctly.

With this change, tx will write a vsindex before the first blend if the vsindex is not zero.

[readroberts]

This looks good to me. I see that the logic adds the vsindex argument and operator, if non-zero, before the first blend operator and its arguments, which matches OpenType spec. The test files changes as expected