fix #269845 Svg don't inheirt QPaintEnginePrivate

SvgPaintEnginePrivate class has always extended QPaintEnginePrivate.  But QPaintEnginePrivate was defined in a header file which was copy-and-pasted from Qt over 6 years ago in the initial git commit.  But with 5.10, Qt added another QRegion member to their definition of QPaintEnginePrivate to fix a bug, so the old copy-and-pasted header no longer correctly represents QPaintEnginePrivate.  This conflicting definition results in a segfault whenever I tried to run mscore 2.1, 2.2, or master at least on my armv7 arch linux machine when using Qt 5.10 (although worked with Qt 5.9).

Lasconic suggested maybe try removing the inheiritance all together.  Turns out SvgPaintEnginePrivate doesn't actually need to inheirit QPaintEnginePrivate.  So the fix here is to remove that inheiritance and complety remove the old copy-and-pasted private header file.

mscore/svggenerator.cpp

@@ -40,7 +40,6 @@
 ****************************************************************************/
 
 #include "svggenerator.h"
-#include "paintengine_p.h"
 #include "libmscore/mscore.h"
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -153,7 +152,7 @@ static QString getClass(const Ms::Element *e)
     return eName;
 }
 
-class SvgPaintEnginePrivate : public QPaintEnginePrivate
+class SvgPaintEnginePrivate
 {
 public:
     SvgPaintEnginePrivate()
@@ -232,6 +231,7 @@ class SvgPaintEngine : public QPaintEngine
 private:
     QString     stateString;
     QTextStream stateStream;
+    SvgPaintEnginePrivate *d_ptr;
 
 // Qt translates everything. These help avoid SVG transform="translate()".
     qreal _dx;
@@ -312,10 +312,10 @@ class SvgPaintEngine : public QPaintEngine
 
 public:
     SvgPaintEngine()
-        : QPaintEngine(*new SvgPaintEnginePrivate,
-                       svgEngineFeatures()),
+        : QPaintEngine(svgEngineFeatures()),
           stateStream(&stateString)
     {
+        d_ptr = new SvgPaintEnginePrivate;
     }
 
     bool begin(QPaintDevice *device);