Fix setting size of wxGenericStaticBitmap

We must call SetInitialSize() in Create() in order to take the size
argument passed to ctor/Create() into account -- it was completely
ignored previously.

But calling SetBitmap() must not change the initial size, as it can also
be done later, so just change the current size there instead (which is
consistent with wxMSW version and original behaviour, so keep it like
this, even though it's not totally clear if all ports do it).

Remove the now unused wxGenericStaticBitmap::GetBitmapSize().

src/generic/statbmpg.cpp

@@ -34,7 +34,13 @@ bool wxGenericStaticBitmap::Create(wxWindow *parent, wxWindowID id,
                             wxDefaultValidator, name))
         return false;
     m_scaleMode = Scale_None;
-    SetBitmap(bitmap);
+
+    // Don't call SetBitmap() here, as it changes the size and refreshes the
+    // window unnecessarily, when we don't need either of these side effects
+    // here.
+    m_bitmap = bitmap;
+    SetInitialSize(size);
+
     Bind(wxEVT_PAINT, &wxGenericStaticBitmap::OnPaint, this);
     return true;
 }