Add support for colored and styled underlines on tty frames

* src/dispextern.h (face, face_underline_type, syms_of_xfacse)
(internal-set-lisp-face-attribute)
(gui_supports_face_attributes_p): Add definitions for new
underline styles of Double-line, Dots and Dashes.  Rename
FACE_UNDER_LINE and FACE_UNDER_WAVE to make definitions
consistent.  Delete tty_underline_p from the face struct and use
just underline going forward.  Add a flag to check whether styled
underlines are available.
* lisp/cus-face.el (custom-face-attributes): Add entries for
Double-line, Dots and Dashes so they can be set through
`customize'.
* src/termchar.c (tty_display_info): Add an entry for the escape
sequence to set the underline style and color on terminal frames.
* src/term.c (init_tty, tty_capable_p, turn_on_face): Read and
save the underline style escape sequence from the Smulx termcap
(alternatively if the Su flag is set use a default sequence).
Allow checking for support of styled underlines in the current
terminal frame.  Output the necessary escape sequences to activate
a styled underline on turn_on_face; this is currently only used
for the new special underline styles, a default straight underline
will still use the "us" termcap.  Output escape sequence to set
underline color when set in the face and supported by the tty.
Save a default value for this sequence on init_tty when styled
underlines are supported.
* src/xfaces.c (tty_supports_face_attributes_p, realize_tty_face)
(map_tty_color): Assert whether styled underlines are supported by
the current terminal on display-supports-face-attributes-p checks.
Populate the correct underline style and color in the face spec
when realizing a face.  Allow map_tty_color to map underline
colors alongside foreground and background.  The interface of
map_tty_color was amended to allow the caller to supply the
underline color instead of accessing it through the face
attributes.  (bug#62994)
* src/xterm.c (x_draw_glyph_string): Updated to use renamed
FACE_UNDERLINE_SINGLE and FACE_UNDERLINE_WAVE face_underline_type
enumerations.

1 files changed, 9 insertions, 6 deletions

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index fba15578f4f..8425aa23422 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -2685,12 +2685,15 @@ Underline in color @var{color}, a string specifying a color.
 @var{color} is either a string, or the symbol @code{foreground-color},
 meaning the foreground color of the face.  Omitting the attribute
 @code{:color} means to use the foreground color of the face.
-@var{style} should be a symbol @code{line} or @code{wave}, meaning to
-use a straight or wavy line.  Omitting the attribute @code{:style}
-means to use a straight line.  @var{position}, if non-@code{nil}, means to
-display the underline at the descent of the text, instead of at the
-baseline level.  If it is a number, then it specifies the amount of
-pixels above the descent to display the underline.
+@var{style} is a symbol which sets the line-style to of the underline.
+It should be one of @code{line}, @code{double-line}, @code{wave},
+@code{dots}, or @code{dashes}.  GUI frames only support @code{line} and
+@code{wave}.  Terminal frames can support all aforementioned underline
+styles.  Omitting the attribute @code{:style} means to use a straight
+line.  @var{position}, if non-@code{nil}, means to display the underline
+at the descent of the text, instead of at the baseline level.  If it is
+a number, then it specifies the amount of pixels above the descent to
+display the underline.
 @end table
 
 @cindex overlined text