write coordinates in C locale

Since a6276cd2 we use g_snprintf for coordinate string formatting. This
function uses number formatting dependant on the current locale and
therefore might produce invalid files. This is fixed now.

Fixes: #1393

src/control/xml/XmlPointNode.cpp

@@ -49,9 +49,7 @@ void XmlPointNode::writeOut(OutputStream* out)
 			out->write(" ");
 		}
 
-		gchar coordinateStr[Util::PRECISION_FORMAT_BUFF_LEN];
-		Util::getCoordinateString(coordinateStr, Util::PRECISION_FORMAT_BUFF_LEN, p->x, p->y);
-		out->write(coordinateStr);
+		Util::writeCoordinateString(out, p->x, p->y);
 	}
 
 	out->write("</");

src/control/xml/XmlStrokeNode.cpp

@@ -89,15 +89,12 @@ void XmlStrokeNode::writeOut(OutputStream* out)
 	{
 		out->write(">");
 
-		gchar coordinateStr[Util::PRECISION_FORMAT_BUFF_LEN];
-		Util::getCoordinateString(coordinateStr, Util::PRECISION_FORMAT_BUFF_LEN, points[0].x, points[0].y);
-		out->write(coordinateStr);
+		Util::writeCoordinateString(out, points[0].x, points[0].y);
 
 		for (int i = 1; i < this->pointsLength; i++)
 		{
-			Util::getCoordinateString(coordinateStr, Util::PRECISION_FORMAT_BUFF_LEN, points[i].x, points[i].y);
 			out->write(" ");
-			out->write(coordinateStr);
+			Util::writeCoordinateString(out, points[i].x, points[i].y);
 		}
 
 		out->write("</");

src/util/Util.cpp

@@ -170,8 +170,12 @@ gboolean Util::paintBackgroundWhite(GtkWidget* widget, cairo_t* cr, void*)
 	return false;
 }
 
-gchar* Util::getCoordinateString(gchar* buff, gulong buffLen, double xVal, double yVal)
+void Util::writeCoordinateString(OutputStream* out, double xVal, double yVal)
 {
-	g_snprintf(buff, buffLen, Util::PRECISION_FORMAT_STRING_XY, xVal, yVal);
-	return buff;
-}
+	char coordString[G_ASCII_DTOSTR_BUF_SIZE];
+	g_ascii_formatd(coordString, G_ASCII_DTOSTR_BUF_SIZE, Util::PRECISION_FORMAT_STRING, xVal);
+	out->write(coordString);
+	out->write(" ");
+	g_ascii_formatd(coordString, G_ASCII_DTOSTR_BUF_SIZE, Util::PRECISION_FORMAT_STRING, yVal);
+	out->write(coordString);
+}

src/util/Util.h

@@ -11,6 +11,7 @@
 
 #pragma once
 
+#include "OutputStream.h"
 #include "Path.h"
 
 #include <gtk/gtk.h>
@@ -55,17 +56,12 @@ gboolean paintBackgroundWhite(GtkWidget* widget, cairo_t* cr, void* unused);
 
 /**
  * Format coordinates to use 8 digits of precision https://m.xkcd.com/2170/
- * This function works like g_ascii_formatd in that it stores the result in buff, and
- * also returns the result.
+ * This function directy writes to the given OutputStream.
  */
-extern gchar* getCoordinateString(gchar* buff, gulong buffLen, double xVal, double yVal);
+extern void writeCoordinateString(OutputStream* out, double xVal, double yVal);
 
 constexpr const gchar* PRECISION_FORMAT_STRING = "%.8f";
 
-constexpr const gchar* PRECISION_FORMAT_STRING_XY = "%.8f %.8f";  // note the space delimiter
-
-constexpr const int PRECISION_FORMAT_BUFF_LEN = G_ASCII_DTOSTR_BUF_SIZE * 2 + 1;
-
 }  // namespace Util
 
-static const size_t npos = std::numeric_limits<size_t>::max();
+static const size_t npos = std::numeric_limits<size_t>::max();