Fix empty line rendering, 2nd try.

Our first attempt in rev. 2f7b346 didn't quite work out because the
tspans were placed in slightly different y positions than before. So
we're back to relative baseline offsets now, and use the "hidden tspan"
trick from this article to make empty lines render correctly:

https://stackoverflow.com/a/58429593

Thanks to stackoverflow member Nate_C for this neat idea!

pd/nw/pdgui.js

@@ -3796,14 +3796,17 @@ function text_to_tspans(cid, svg_text, text) {
     fontsize = svg_text.getAttribute("font-size").slice(0, -2);
     var dy = text_line_height_kludge(+fontsize, "gui");
     for (i = 0; i < len; i++) {
-        // ag: We need to specify absolute y offsets for the baseline
-        // positions here. Relative dy offsets, which were used previously,
-        // don't advance the y position if no glyphs are rendered, so empty
-        // lines aren't rendered correctly.
         tspan = create_item(cid, "tspan", {
-            y: ((i+1)*dy) + "px",
+            dy: i==0 ? 0 : dy + "px",
             x: 0
         });
+        if (/^\s*$/.test(lines[i])) {
+            // ag: empty spans won't render correctly, so add something other
+            // than a blank and hide the contents of the tspan. See
+            // https://stackoverflow.com/a/58429593
+            tspan.style.visibility = "hidden"
+            lines[i] = ".";
+        }
         // find a way to abstract away the canvas array and the DOM here
         text_node = patchwin[cid].window.document
                     .createTextNode(lines[i]);