Remove unnecessary sixel sequence wrapper.

[j4james]

The sixel_encode function will already generate a complete Sixel
sequence, so there is no need to wrap that with an additional sequence
introducer and terminator, and doing so can cause problems on some
terminals.

[j4james]

I've tested this on 16 different Sixel terminals, and I've confirmed that it still works as well as before (and in some cases better) than without the PR applied.

Quite a few terminals (MLTerm, St, Yaft, RLogin) had glitches in the first few scanlines of the image which are no longer apparent. The Rxvt sixel fork just used to spew the entire sequence contents as text, and that's now fixed. And as I mentioned in #145, Windows Terminal would previously fill the background with a random color, and that's no longer the case.

[hzeller]

Thanks!