Fix storing wxBitmap data in GdkPixbuf

Under wxGTK+2 bitmap data with mask and without it (raw) should be stored
in the separate GdkPixbuf buffers - just like it's done in wxGTK+3. These
two buffers are necessary because only GdkPixbuf with raw bitmap data
(original, non-masked) should be copied when wxBitmapRefData instance is
cloned e.g. in SetMask(). GdkPixbuf with masked data is not copied and is
created on first use in wxBitmap::GetPixbuf().

Closes #18508.
See #18498.

include/wx/gtk/bitmap.h

@@ -133,7 +133,6 @@ public:
 #endif
 
 #ifdef __WXGTK3__
-    GdkPixbuf* GetPixbufNoMask() const;
     cairo_t* CairoCreate() const;
     void Draw(cairo_t* cr, int x, int y, bool useMask = true, const wxColour* fg = NULL, const wxColour* bg = NULL) const;
     void SetSourceSurface(cairo_t* cr, int x, int y, const wxColour* fg = NULL, const wxColour* bg = NULL) const;
@@ -143,6 +142,7 @@ public:
     bool HasPixbuf() const;
     wxBitmap(GdkPixmap* pixmap);
 #endif
+    GdkPixbuf* GetPixbufNoMask() const;
     GdkPixbuf *GetPixbuf() const;
 
     // raw bitmap access support functions