Don't use implicit wxBitmapRefData copy ctor in wxGTK code.

wxBitmapRefData didn't have a proper copy ctor but the code in wxGTK wxBitmap
implementation used it nevertheless and then manually patched the newly copied
object to avoid double pointer deletion and other unpleasantness.

Make the code more obviously correct by not using (nor providing) copy ctor at
all. There are no real changes otherwise.

See #12768.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66372 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775