Fix multiline vertical text positioning in browsers.

Fix #1227 (closed)

This solution assumes that we should modify the layout so that public functions return x/y values taking this into account.

~~Note: one undocumented change, when Alignment == NONE in chunkAnchorPoint, we no longer add chunk_width * 0.5 to the anchor point's x value. That now matches _getChunkLeftWithAlignment~~Update: looking into uses of NONE, I think Alignment NONE might occur when text is empty, in which case there is no change in behavior.

I'm not sure this is the right approach. An alternate approach that doesn't touch Layout-TNG:

diff --git a/src/object/sp-text.cpp b/src/object/sp-text.cpp
index 935446c54a..6c81c06314 100644
--- a/src/object/sp-text.cpp
+++ b/src/object/sp-text.cpp
@@ -893,6 +893,9 @@ void SPText::rebuildLayout()
                  && tspan->attributes.singleXYCoordinates() ) {
                 Inkscape::Text::Layout::iterator iter = layout.sourceToIterator(tspan);
                 Geom::Point anchor_point = layout.chunkAnchorPoint(iter);
+                if (!is_horizontal()) {
+                    std::swap(anchor_point[Geom::X], anchor_point[Geom::Y]);
+                }
                 tspan->attributes.setFirstXY(anchor_point);
                 // repr needs to be updated but if we do it here we get a loop.
             }

and

diff --git a/src/object/sp-flowtext.cpp b/src/object/sp-flowtext.cpp
index af675eeeb4..1238254634 100644
--- a/src/object/sp-flowtext.cpp
+++ b/src/object/sp-flowtext.cpp
@@ -496,11 +496,16 @@ Inkscape::XML::Node *SPFlowtext::getAsText()
         return nullptr;
     }
 
+    bool horizontal = (style->writing_mode.computed == SP_CSS_WRITING_MODE_LR_TB || style->writing_mode.computed == SP_CSS_WRITING_MODE_RL_TB);
+
     Inkscape::XML::Document *xml_doc = this->document->getReprDoc();
     Inkscape::XML::Node *repr = xml_doc->createElement("svg:text");
     repr->setAttribute("xml:space", "preserve");
     repr->setAttribute("style", this->getRepr()->attribute("style"));
     Geom::Point anchor_point = this->layout.characterAnchorPoint(this->layout.begin());
+    if (!horizontal) {
+        std::swap(anchor_point[Geom::X], anchor_point[Geom::Y]);
+    }
     repr->setAttributeSvgDouble("x", anchor_point[Geom::X]);
     repr->setAttributeSvgDouble("y", anchor_point[Geom::Y]);
 
@@ -515,6 +520,9 @@ Inkscape::XML::Node *SPFlowtext::getAsText()
 
             Inkscape::XML::Node *span_tspan = xml_doc->createElement("svg:tspan");
             Geom::Point anchor_point = this->layout.characterAnchorPoint(it);
+            if (!horizontal) {
+                std::swap(anchor_point[Geom::X], anchor_point[Geom::Y]);
+            }
             // use kerning to simulate justification and whatnot
             Inkscape::Text::Layout::iterator it_span_end = it;
             it_span_end.nextStartOfSpan();