csi: implement SGR 21, double underlines

CHANGELOG.md

@@ -62,6 +62,7 @@
   reduces the memory usage, but should also be slightly faster.
 * Support for high-res mouse wheel scroll events ([#1738][1738]).
 * Styled and colored underlines ([#828][828]).
+* Support for SGR 21 (double underline).
 
 [1707]: https://codeberg.org/dnkl/foot/issues/1707
 [1738]: https://codeberg.org/dnkl/foot/issues/1738

csi.c

@@ -129,7 +129,13 @@ csi_sgr(struct terminal *term)
         case 8: term->vt.attrs.conceal = true; break;
         case 9: term->vt.attrs.strikethrough = true; break;
 
-        case 21: break; /* double-underline, not implemented */
+        case 21:
+            term->vt.attrs.underline = true;
+            term->vt.underline.style = UNDERLINE_DOUBLE;
+            term->bits_affecting_ascii_printer.underline_style = true;
+            term_update_ascii_printer(term);
+            break;
+
         case 22: term->vt.attrs.bold = term->vt.attrs.dim = false; break;
         case 23: term->vt.attrs.italic = false; break;
         case 24: {

doc/foot-ctlseqs.7.scd

@@ -159,6 +159,8 @@ m*.
 :  Conceal; text is not visible, but is copiable
 |  9
 :  Crossed-out/strike
+|  21
+:  Double underline
 |  22
 :  Disable *bold* and *dim*
 |  23