[sixel]: re-enable Set Raster Attributes, and fix the ‘pad’ parameter

Now that we explicitly set P2=1, we can safely emit a "Set Raster
Attributes" command, to tell the terminal the final size of the image
up front.

Also fix the ‘pad’ parameter (horizontal aspect ratio); XTerm rejects
sixels with pan or pad set to 0.

src/lib/sixel.c

@@ -322,7 +322,7 @@ write_sixel_data(FILE* fp, int leny, int lenx, const sixeltable* stab, int* pars
   // Set Raster Attributes - pan/pad=1 (pixel aspect ratio), Ph=lenx, Pv=leny
   // using Ph/Pv causes a background to be drawn using color register 0 for all
   // unspecified pixels, which we do not want.
-  //fprintf(fp, "\"1;0;%d;%d", lenx, leny);
+  fprintf(fp, "\"1;1;%d;%d", lenx, leny);
   (void)leny;
 
   for(int i = 0 ; i < stab->colors ; ++i){