tty: handle indexed colors separately from RGB color space

The terminfo entries from the standard ncurses distribution have
peculiar settings for entries supporting 24 bit colors. The direct
entries mix indexed and RGB values into an incompatible mess.

This commit adds a simple workaround for the tty port.  Colors are
initialised as if only 8 ANSI colors are available.  This does not
affect color customisation from the symsets.

The curses port is affected as well. But I am not yet comfortable to
refactor a large part of the code for an absolute edge case.

win/tty/termcap.c

@@ -31,6 +31,7 @@ static void analyze_seq(char *, int *, int *);
 #if (defined(TERMLIB) || defined(ANSI_DEFAULT))
 static void init_hilite(void);
 static void kill_hilite(void);
+static int indexed_color_count(void);
 #endif
 
 /* (see tcap.h) -- nh_CM, nh_ND, nh_CD, nh_HI,nh_HE, nh_US,nh_UE, ul_hack */
@@ -947,6 +948,15 @@ init_hilite(void)
 
     colors = tgetnum(nhStr("Co"));
     iflags.colorcount = colors;
+#ifdef NCURSES_VERSION
+    /* The standard ncurses terminfo entries that support 24-bit colors
+       (*-direct) map the standard 8/16/256 colors onto the beginning
+       of the 24-bit color range. For example, rgb(0,0,1) appears as red
+       instead of nearly black. iflags.colorcount should retain the actual
+       supported color count, while for default color initialization we
+       take the available indexed colors into consieration. */
+    colors = indexed_color_count();
+#endif
     int md_len = 0;
 
     if (colors < 8 || !MD || !*MD
@@ -1054,7 +1064,7 @@ kill_hilite(void)
         if (hilites[CLR_BLACK] != hilites[CLR_BLUE])
             free(hilites[CLR_BLACK]), hilites[CLR_BLACK] = NULL;
     }
-    if (tgetnum(nhStr("Co")) >= 16) {
+    if (indexed_color_count() >= 16) {
         if (hilites[CLR_BLUE])
             free(hilites[CLR_BLUE]);
         if (hilites[CLR_GREEN])
@@ -1096,6 +1106,21 @@ kill_hilite(void)
         hilites[c] = NULL;
 }
 
+static int
+indexed_color_count(void)
+{
+#ifdef NCURSES_VERSION
+    /* ncurses adds the non-standard attribute CO for "number of indexed
+       colors overlaying RGB space". */
+    int idx_colors = tgetnum(nhStr("CO"));
+    if (idx_colors > 0) {
+        return idx_colors;
+    }
+#endif
+
+    return tgetnum(nhStr("Co"));
+}
+
 #else /* UNIX && TERMINFO */
 
 #ifndef TOS