graphics/lsix: update to 1.8

Fix scrolling for genuine VT340, foot, and other terminals

After purchasing a genuine DEC VT340 terminal, @hackerb9 has learned that the
DEC VT340 graphics programmer's manual was mistaken and the sense of DECSDM
should be inverted.
Nearly every terminal emulator which built upon that reference over the last
thirty years is incorrect. Some are correcting the error now, some never were
wrong, but we are at a time when it is not safe to presume what setting DECSDM
means.

For now, lsix will simply presume that sixel scrolling is enabled and hope that
no program has been run before it that disables it.

See #41 for details.

Thanks go to @j4james for pointing out the mistake and for testing multiple
terminal emulators to show that none of them currently default to having sixel
scrolling disabled.

(pin)