support ligatures display while being input (mintty/wsltty#123, #601)

docs/mintty.1

@@ -1425,6 +1425,13 @@ The RowSpacing value is added to that.
 .br
 (Corresponds roughly to the xterm resource \fBscaleHeight\fP.)
 
+.TP
+\fBLigatures support\fP (LigaturesSupport=0)
+By default, ligatures, as supported by the selected font, are rendered 
+if they are output to the terminal in one chunk. When this option is 
+set =1, mintty redisplays the left part of the line whenever a character 
+is output, so ligatures are also supported while being input.
+
 .TP
 \fB\fP(ColSpacing=0)
 Additional column padding; ColSpacing=1 can avoid boldened glyphs being clipped.

src/config.c

@@ -176,6 +176,7 @@ const config default_cfg = {
   .col_spacing = 0,
   .row_spacing = 0,
   .padding = 1,
+  .ligatures_support = 0,
   .handle_dpichanged = true,
   .check_version_update = 900,
   .word_chars = "",
@@ -403,6 +404,7 @@ options[] = {
   {"ColSpacing", OPT_INT, offcfg(col_spacing)},
   {"RowSpacing", OPT_INT, offcfg(row_spacing)},
   {"Padding", OPT_INT, offcfg(padding)},
+  {"LigaturesSupport", OPT_INT, offcfg(ligatures_support)},
   {"HandleDPI", OPT_BOOL, offcfg(handle_dpichanged)},
   {"CheckVersionUpdate", OPT_INT, offcfg(check_version_update)},
   {"WordChars", OPT_STRING, offcfg(word_chars)},

src/config.h

@@ -174,6 +174,7 @@ typedef struct {
   int geom_sync;
   int col_spacing, row_spacing;
   int padding;
+  int ligatures_support;
   int handle_dpichanged;
   int check_version_update;
   string word_chars;

src/termout.c

@@ -279,6 +279,8 @@ write_char(wchar c, int width)
     clear_cc(line, curs->x);
     line->chars[curs->x].chr = c;
     line->chars[curs->x].attr = curs->attr;
+    if (cfg.ligatures_support)
+      term_invalidate(0, curs->y, curs->x, curs->y);
   }
 
   if (curs->wrapnext && curs->autowrap && width > 0) {