Sets the doc's fontSize/font/color from a TextStyle — a single place replacing the pattern
that used to be duplicated in text-block/signature-block/anchor/group band (it was once the
source of a color-bleed bug in demo 06); fontFamily unset = use the doc's current font,
color unset = keep the current color (a caller that always wants a black default should
pass color: style.color ?? [0, 0, 0] itself)
drawing text in a custom block: applyTextStyle is drawText's companion (fontSize/font/colour), lineHeightOf tells you how far to advance afterwards