Add sixel support

Originated from https://gist.github.com/CommandMaker/27ba97427424c06aff54d0a2a91a4ef1

config.def.h

@@ -23,7 +23,10 @@ char *scroll = NULL;
 char *stty_args = "stty raw pass8 nl -echo -iexten -cstopb 38400";
 
 /* identification sequence returned in DA and DECID */
-char *vtiden = "\033[?6c";
+char *vtiden = "\033[?62;4c"; /* VT200 family (62) with sixel (4) */
+
+/* sixel rgb byte order: LSBFirst or MSBFirst */
+int const sixelbyteorder = LSBFirst;
 
 /* Kerning / character bounding-box multipliers */
 static float cwscale = 1.0;