Initialize wxGenericImageList members in default ctor

Don't leave m_useMask and m_scaleFactor uninitialized -- even if this
probably doesn't matter, call Create() to make sure they have
well-defined values for the default-constructed objects.

include/wx/generic/imaglist.h

@@ -22,7 +22,7 @@ class WXDLLIMPEXP_FWD_CORE wxColour;
 class WXDLLIMPEXP_CORE wxGenericImageList: public wxObject
 {
 public:
-    wxGenericImageList() { }
+    wxGenericImageList();
     wxGenericImageList( int width, int height, bool mask = true, int initialCount = 1 );
     virtual ~wxGenericImageList();
     bool Create( int width, int height, bool mask = true, int initialCount = 1 );

src/generic/imaglist.cpp

@@ -29,6 +29,11 @@
 wxIMPLEMENT_DYNAMIC_CLASS(wxGenericImageList, wxObject);
 wxIMPLEMENT_DYNAMIC_CLASS(wxImageList, wxGenericImageList);
 
+wxGenericImageList::wxGenericImageList()
+{
+    Create(0, 0, false);
+}
+
 wxGenericImageList::wxGenericImageList( int width, int height, bool mask, int initialCount )
 {
     (void)Create(width, height, mask, initialCount);