Fix wxConvAuto behaviour when it is used by wxTextInputStream.

Fix wxConvAuto behaviour when it is used by wxTextInputStream.

wxConvAuto implicitly supposed that the chunk of data passed to it for
translation was big enough to allow it to at least detect the BOM from it.
However this isn't necessarily the case and never is with wxTextInputStream
which reads the bytes one by one.

Fix this by waiting until we have enough data to be able to detect the BOM.
This still doesn't fix the problem with streams without BOM and the
corresponding unit test still fails -- it will need to be fixed at the level
of wxTextInputStream itself later but handling correctly the cases when a BOM
is present is already better than before.

See #11570.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63064 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775