WebCore::PathQt::platformPath(): don't return a temporary object

`WebCore::Path::platformPath()` expects to be able to return the return
value of `WebCore::PlatformPathImpl::platformPath()` as a
`PlatformPathPtr` without creating a dangling reference. However,
`WebCore::PathQt::platformPath()` is defined as returning a temporary
`QPainterPath` object, which does become a dangling reference when
returned by reference, leading to segfaults at runtime.

This commit changes the return type of `WebCore::PathQt::platformPath()`
to `PlatformPathPtr` (paralleling the definition of
`WebCore::PathCG::platformPath()`), so as to avoid creating a temporary
`QPainterPath` object, thereby avoiding the creation of a dangling
reference when returning from `WebCore::Path::platformPath()`.

Fixes: movableink#37

Source/WebCore/platform/graphics/qt/PathQt.cpp

@@ -110,7 +110,7 @@ Ref<PathImpl> PathQt::copy() const
     return create(m_path);
 }
 
-QPainterPath PathQt::platformPath() const
+PlatformPathPtr PathQt::platformPath() const
 {
     return m_path;
 }

Source/WebCore/platform/graphics/qt/PathQt.h

@@ -52,7 +52,7 @@ class PathQt final : public PathImpl {
     PathQt& operator=(const PathQt&);
     PathQt& operator=(PathQt&& other);
 
-    QPainterPath platformPath() const;
+    PlatformPathPtr platformPath() const;
 
     void addPath(const PathQt&, const AffineTransform&);
 
@@ -68,8 +68,6 @@ class PathQt final : public PathImpl {
 private:
     Ref<PathImpl> copy() const final;
 
-    QPainterPath ensurePlatformPath() { return platformPath(); }
-
     void add(PathMoveTo) final;
 
     void add(PathLineTo) final;