patch 9.1.1078: Terminal ansi colors off by one after tgc reset

patch 9.1.1078: Terminal ansi colors off by one after tgc reset

Problem:  Terminal ansi colors off by one after tgc reset
Solution: Set the correct index for libvterm palette,
          revert parts in libvterm/src/pen.c that deviated from upstream
          (Julio B)

fixes: #16568
closes: #16573
related: Vim patch v8.2.0804

Co-authored-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Julio B <julio.bacel@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>