Fix svg outline widths are incorrectly scaled (fix #11522)

SVG outline width sizes were not correctly calculated, and were
set to a constant value regardless of the rendered size of the svg
image. This meant that:
1. the rendered outline width was effectively randomly scaled
2. the width would change as the symbol size was modified

This change has some large flow on effects, eg:

- the large outline widths required to render an outline in <2.12
will now be drawn in their correct sizes, eg massive outlines. Projects
will need to be updated to reflect this.
- the default outline width set for the provided svg symbols (1 mm)
is much too large, so the symbols look bad with the default width.
This size needs to be modified in all the (~300 svg marker images)
- On the plus side, the long standing issue where most of the svg
symbols were rendered with white fill on white background and an
almost invisible black border is now fixed (refs #10908, #8538)

src/core/symbology-ng/qgsmarkersymbollayerv2.cpp

@@ -1039,7 +1039,7 @@ QgsSvgMarkerSymbolLayerV2::QgsSvgMarkerSymbolLayerV2( const QString& name, doubl
   mAngle = angle;
   mOffset = QPointF( 0, 0 );
   mScaleMethod = scaleMethod;
-  mOutlineWidth = 1.0;
+  mOutlineWidth = 0.2;
   mOutlineWidthUnit = QgsSymbolV2::MM;
   mColor = QColor( Qt::black );
   mOutlineColor = QColor( Qt::black );

src/core/symbology-ng/qgssvgcache.cpp

@@ -229,7 +229,7 @@ void QgsSvgCache::containsParams( const QString& path, bool& hasFillParam, QColo
 {
   defaultFillColor = QColor( Qt::black );
   defaultOutlineColor = QColor( Qt::black );
-  defaultOutlineWidth = 1.0;
+  defaultOutlineWidth = 0.2;
 
   QDomDocument svgDoc;
   if ( !svgDoc.setContent( getImageData( path ) ) )
@@ -256,12 +256,56 @@ void QgsSvgCache::replaceParamsAndCacheSvg( QgsSvgCacheEntry* entry )
 
   //replace fill color, outline color, outline with in all nodes
   QDomElement docElem = svgDoc.documentElement();
-  replaceElemParams( docElem, entry->fill, entry->outline, entry->outlineWidth );
+
+  double sizeScaleFactor = calcSizeScaleFactor( entry, docElem );
+  replaceElemParams( docElem, entry->fill, entry->outline, entry->outlineWidth * sizeScaleFactor );
 
   entry->svgContent = svgDoc.toByteArray();
   mTotalSize += entry->svgContent.size();
 }
 
+double QgsSvgCache::calcSizeScaleFactor( QgsSvgCacheEntry* entry, const QDomElement& docElem ) const
+{
+  QString viewBox;
+
+  //bad size
+  if ( !entry || entry->size == 0 )
+    return 1.0;
+
+  //find svg viewbox attribute
+  //first check if docElem is svg element
+  if ( docElem.tagName() == "svg" )
+  {
+    viewBox = docElem.attribute( "viewBox", QString() );
+  }
+  else
+  {
+    QDomElement svgElem = docElem.firstChildElement( "svg" ) ;
+    if ( !svgElem.isNull() )
+    {
+      viewBox = svgElem.attribute( "viewBox", QString() );
+    }
+  }
+
+  //could not find valid viewbox attribute
+  if ( viewBox.isEmpty() )
+    return 1.0;
+
+  //width should be 3rd element in a 4 part space delimited string
+  QStringList parts = viewBox.split( " " );
+  if ( parts.count() != 4 )
+    return 1.0;
+
+  bool widthOk = false;
+  double width = parts.at( 2 ).toDouble( &widthOk );
+  if ( widthOk )
+  {
+    return width / entry->size ;
+  }
+
+  return 1.0;
+}
+
 QByteArray QgsSvgCache::getImageData( const QString &path ) const
 {
   // is it a path to local file?

src/core/symbology-ng/qgssvgcache.h

@@ -178,6 +178,9 @@ class CORE_EXPORT QgsSvgCache : public QObject
     void containsElemParams( const QDomElement& elem, bool& hasFillParam, QColor& defaultFill, bool& hasOutlineParam, QColor& defaultOutline,
                              bool& hasOutlineWidthParam, double& defaultOutlineWidth ) const;
 
+    /** Calculates scaling for rendered image sizes to SVG logical sizes*/
+    double calcSizeScaleFactor( QgsSvgCacheEntry* entry, const QDomElement& docElem ) const;
+
     /** Release memory and remove cache entry from mEntryLookup*/
     void removeCacheEntry( const QString& s, QgsSvgCacheEntry* entry );
 
@@ -189,6 +192,7 @@ class CORE_EXPORT QgsSvgCache : public QObject
 
     //! Mutex to prevent concurrent access to the class from multiple threads at once (may corrupt the entries otherwise).
     QMutex mMutex;
+
 };
 
 #endif // QGSSVGCACHE_H

src/ui/symbollayer/widget_svgmarker.ui

@@ -35,6 +35,9 @@
          <property name="singleStep">
           <double>0.200000000000000</double>
          </property>
+         <property name="value">
+          <double>0.200000000000000</double>
+         </property>
          <property name="showClearButton" stdset="0">
           <bool>false</bool>
          </property>