DRCS-SIXEL version 2

<DECDLD> = DCS Pfn; Pcn; Pe; Pcmw; Pss; 3; Pcmh; Pcss { SP Dscs <SIXEL> ST
<SIXEL> = P1; P2; P3; q s...s

 * P1; P2; P3; q is optional.
 * " Pan; Pad; Ph; Pv in s...s is optional.

 This sequence devides sixel graphics image by cell(*1), and defines image
pieces as the glyphs of <Pcn+0x20+N>(*2) code of <Dscs> character set in order
from the top-left corner cell.

(*1) The number of columns and rows are calculated as follows.
 <columns> = sixel graphics image width / cell width (rounded up)
 <rows> = sixel graphics image height / cell height (rounded up)
 (Width and height include transparent pixels)

(*2) 0x20 of <Dscs+1> is next to 0x7f of <Dscs>, and 0x20 of 0x30 set
 which consists of 96 characters is next to 0x7f of 0x7e set which
 consists of 94 characters.

(*3) P2=1 (Pixel positions specified as 0 remain at their current color) is
 not supported for now.

[Appendix]
<Original DRCS-SIXEL (rlogin 2.23.0 or before)>
o The number of columns and rows of sixel graphic image are calculated
  as follows.
  <columns> = sixel graphics image width / cell width (rounded down)
  <rows> = sixel graphics image height / cell height (rounded down)
  (Width and height excludes transparent pixels)

o The range of defining image pieces as font glyphs by DECDLD is
  <Pcn+0x20> - 0x7f of <Dscs> alone.

<DRCS mapping rule version 2>
U+10XXYY (0x30 <= 0xXX <= 0x7E, 0x20 <= 0xYY <= 0x7F)
 => ESC ( SP <\xXX> <\xYY> ESC ( B

U+10XXZZ (0x30 <= 0xXX <= 0x7E, 0xA0 <= 0xZZ <= 0xFF)
 => ESC - SP <\xXX> <\xZZ>

The width of U+10XXYY and U+10XXZZ (PUA) is defined as ambiguous in
EastAsianWidth.txt, but all of terminal emulators and console applications
should regard it as halfwidth to show DRCS-SIXEL correctly.

(DRCS mapping rule version 1: https://github.com/saitoha/drcsterm/blob/master/README.rst)
