Add true color support.

Implement true color support define in:
https://en.wikipedia.org/wiki/ANSI_escape_code

The sequence is:
ESC[ … 38;2;;; … m Select RGB foreground color
ESC[ … 48;2;;; … m Select RGB background color

src/terminal/terminalframebuffer.cc

@@ -505,6 +505,8 @@ void Renditions::set_foreground_color( int num )
 {
   if ( (0 <= num) && (num <= 255) ) {
     foreground_color = 30 + num;
+  } else if ( is_true_color( num ) ) {
+    foreground_color = num;
   }
 }
 
@@ -512,6 +514,8 @@ void Renditions::set_background_color( int num )
 {
   if ( (0 <= num) && (num <= 255) ) {
     background_color = 40 + num;
+  } else if ( is_true_color( num ) ) {
+    background_color = num;
   }
 }
 
@@ -544,13 +548,27 @@ std::string Renditions::sgr( void ) const
 
   ret.append( "m" );
 
-  if ( foreground_color > 37 ) { /* use 256-color set */
+  if ( is_true_color( foreground_color ) ) {
+    char col[64];
+    snprintf( col, 64, "\033[38;2;%d;%d;%dm",
+              (foreground_color >> 16) & 0xff,
+              (foreground_color >> 8) & 0xff,
+              foreground_color & 0xff);
+    ret.append( col );
+  } else if ( foreground_color > 37 ) { /* use 256-color set */
     char col[ 64 ];
     snprintf( col, 64, "\033[38;5;%dm", foreground_color - 30 );
     ret.append( col );
   }
 
-  if ( background_color > 47 ) { /* use 256-color set */
+  if ( is_true_color( background_color ) ) {
+    char col[64];
+    snprintf( col, 64, "\033[48;2;%d;%d;%dm",
+              (background_color >> 16) & 0xff,
+              (background_color >> 8) & 0xff,
+              background_color & 0xff);
+    ret.append( col );
+  } else if ( background_color > 47 ) { /* use 256-color set */
     char col[ 64 ];
     snprintf( col, 64, "\033[48;5;%dm", background_color - 40 );
     ret.append( col );

src/terminal/terminalframebuffer.h

@@ -55,9 +55,9 @@ namespace Terminal {
   public:
     typedef enum { bold, faint, italic, underlined, blink, inverse, invisible, SIZE } attribute_type;
 
-    // all together, a 32 bit word now...
-    unsigned int foreground_color : 12;
-    unsigned int background_color : 12;
+    static const unsigned int true_color_mask = 0x80000000;
+    unsigned int foreground_color;
+    unsigned int background_color;
   private:
     unsigned int attributes : 8;
 
@@ -68,6 +68,14 @@ namespace Terminal {
     void set_rendition( color_type num );
     std::string sgr( void ) const;
 
+    static unsigned int make_true_color( unsigned int r, unsigned int g, unsigned int b ) {
+      return true_color_mask | (r << 16) | (g << 8) | b;
+    }
+
+    static bool is_true_color(unsigned int color) {
+      return (color & true_color_mask) != 0;
+    }
+
     bool operator==( const Renditions &x ) const
     {
       return ( attributes == x.attributes )

src/terminal/terminalfunctions.cc

@@ -424,6 +424,27 @@ static void CSI_SGR( Framebuffer *fb, Dispatcher *dispatch )
       i += 2;
       continue;
     }
+
+    /* True color support: ESC[ ... [34]8;2;<r>;<g>;<b> ... m */
+    if ( (rendition == 38 || rendition == 48) &&
+         (dispatch->param_count() - i >= 5) &&
+         (dispatch->getparam( i+1, -1 ) == 2)) {
+      unsigned int red = dispatch->getparam(i+2, 0);
+      unsigned int green = dispatch->getparam(i+3, 0);
+      unsigned int blue = dispatch->getparam(i+4, 0);
+      unsigned int color;
+
+      color = Renditions::make_true_color( red, green, blue );
+
+      if ( rendition == 38 ) {
+        fb->ds.set_foreground_color( color );
+      } else {
+        fb->ds.set_background_color( color );
+      }
+      i += 4;
+      continue;
+    }
+
     fb->ds.add_rendition( rendition );
   }
 }