hdspmixer: add output loopback buttons

This adds "LPBK" buttons to the output strip
channels for toggling the hardware output
loopback functionality recently added for
HDSP9632.

The preset data is just appended to the
preset file so that it can still be loaded
in older versions.

hdspmixer/pixmaps/loopback.xpm

@@ -0,0 +1,69 @@
+/* XPM */
+char const * loopback_xpm[] = {
+"34 15 51 1",
+" 	c None",
+".	c #2E3038",
+"+	c #FFFB7C",
+"@	c #FDF97B",
+"#	c #2E2D11",
+"$	c #000000",
+"%	c #383716",
+"&	c #898740",
+"*	c #F1EE75",
+"=	c #22210A",
+"-	c #727034",
+";	c #E2DE6D",
+">	c #D5D267",
+",	c #787637",
+"'	c #FBF77A",
+")	c #AAA750",
+"!	c #ABA851",
+"~	c #CDC962",
+"{	c #83813C",
+"]	c #C8C560",
+"^	c #84823D",
+"/	c #F7F378",
+"(	c #949145",
+"_	c #B7B457",
+":	c #949245",
+"<	c #353414",
+"[	c #87853F",
+"}	c #F1ED75",
+"|	c #3B3A17",
+"1	c #DFDB6C",
+"2	c #161505",
+"3	c #4B4A20",
+"4	c #E5E16E",
+"5	c #FEFA7C",
+"6	c #E8E470",
+"7	c #535124",
+"8	c #FAF679",
+"9	c #E7E370",
+"0	c #66642E",
+"a	c #E4E06E",
+"b	c #F6F277",
+"c	c #EAE671",
+"d	c #E0DC6C",
+"e	c #B9B658",
+"f	c #5F5D2A",
+"g	c #EDE973",
+"h	c #69672F",
+"i	c #DDD96A",
+"j	c #333342",
+"k	c #32323E",
+"l	c #313136",
+"..................................",
+".++++++++++++++++++++++++++++++++.",
+".++++++++++++++++++++++++++++++++.",
+".++++++++++++++++++++++++++++++++.",
+".+++@#++++@$$%&*@$$=-;+@#++>,*+++.",
+".+++@#++++@#+')!@#+@~{+@#+]^/++++.",
+".+++@#++++@#+'!!@#+@~(+@#_:'+++++.",
+".+++@#++++@$$<[}@$$$|1+@234++++++.",
+".+++@#++++@#++++@#+5678@#90a+++++.",
+".+++@#++++@#++++@#+59#b@#+c0d++++.",
+".+++@$$$$e@#++++@$$2f]+@#++ghi+++.",
+".++++++++++++++++++++++++++++++++.",
+".++++++++++++++++++++++++++++++++.",
+".++++++++++++++++++++++++++++++++.",
+"..jklllllllllkj....jklllllllllkj.."};