Render invisible attribute

Implement invisible mode by setting the foreground color to be the same
as the background color. Not rendering anything would also be an
alternative, but this seems less likely to cause surprises in
conjunction with any hacks.

Signed-off-by: Roberto E. Vargas Caballero <k0ga@shike2.com>

st.c

@@ -3236,6 +3236,9 @@ xdraws(char *s, Glyph base, int x, int y, int charlen, int bytelen) {
 	if(base.mode & ATTR_BLINK && term.mode & MODE_BLINK)
 		fg = bg;
 
+	if(base.mode & ATTR_INVISIBLE)
+		fg = bg;
+
 	/* Intelligent cleaning up of the borders. */
 	if(x == 0) {
 		xclear(0, (y == 0)? 0 : winy, borderpx,