Handle deprecated background styles correctly in wxMSW.

Handle deprecated background styles correctly in wxMSW.

Handle wxBG_STYLE_TRANSPARENT as wxBG_STYLE_PAINT and wxBG_STYLE_COLOUR as
wxBG_STYLE_ERASE instead of asserting (and almost surely crashing next, as
this assert is in the code called when the window is repainted and so will be
called again when the assert dialog is shown resulting in reentrant assert).

Closes #11208.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62049 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775