Make color escape sequences conform to terminfo entry

img2xterm had been combining multiple 256color rendition changes in the
same SGR sequence, e.g., `ESC [ 48;5;179; 38;5;100 m` to change both the
background and foreground color.

The xterm-256color terminfo entry requires (and mosh enforces) that
these be in separate escapes. This commit makes the img2xterm output
render correctly in mosh.

Example: `echo -e "setab 179\nsetaf 100" | tput -S | od -c` generates
`ESC [ 48;5;179 m ESC [ 38;5;100 m`.

* Olivia5k/doge#48
* mobile-shell/mosh#519
* mobile-shell/mosh#110 (comment)

README.md

@@ -65,9 +65,7 @@ Converting images
 Known issues
 ------------
 
- * Doesn't work in [mosh] [7].
  * There is something wrong with the implementation of [CIE94 delta-E] [8].
  * ImageMagick can be glitchy. Perhaps using libpng would be a better idea.
 
-[7]: https://github.com/keithw/mosh
 [8]: https://en.wikipedia.org/wiki/Color_difference#CIE94

img2xterm.c

@@ -251,19 +251,12 @@ void bifurcate(FILE* file, unsigned long color1, unsigned long color2, char* bst
 	if (bg != oldbg)
 	{
 		if (bg == color_transparent)
-			fputs(cowheader ? "\\e[49" : "\e[49", file);
+			fputs(cowheader ? "\\e[49m" : "\e[49m", file);
 		else
-			fprintf(file, cowheader ? "\\e[48;5;%lu" : "\e[48;5;%lu", bg);
-
-		if (fg != oldfg)
-			if (fg == color_undef)
-				fputs(";39m", file);
-			else
-				fprintf(file, ";38;5;%lum", fg);
-		else
-			fputc('m', file);
+			fprintf(file, cowheader ? "\\e[48;5;%lum" : "\e[48;5;%lum", bg);
 	}
-	else if (fg != oldfg)
+
+	if (fg != oldfg)
 	{
 		if (fg == color_undef)
 			fputs(cowheader ? "\\e[39m" : "\e[39m", file);