Add dummy wxBitmap::HasAlpha() to wxX11

This allows to write code using this function without any preprocessor
checks.

It should arguably be added to wxBitmapBase to ensure that it's
available in all ports, but should be done together with UseAlpha(),
which is not quite trivial, so leave it for later.

include/wx/x11/bitmap.h

@@ -139,6 +139,10 @@ public:
 
     WXDisplay *GetDisplay() const;
 
+    // This is provided only for compatibility with the other ports, there is
+    // no alpha support in X11 bitmaps.
+    bool HasAlpha() const { return false; }
+
 protected:
     virtual wxGDIRefData *CreateGDIRefData() const;
     virtual wxGDIRefData *CloneGDIRefData(const wxGDIRefData *data) const;