No changes, just refactor wxMSW background brush methods.

No changes, just refactor wxMSW background brush methods.

Factor out MSWGetCustomBgBrush() from MSWGetBgBrushForChild(). This is useful
as in the vast majority of cases the parent window will want to use the same
background brush for all of its children so it doesn't really care about the
concrete child passed to MSWGetBgBrushForChild() and we can adjust the brush
to the child origin in the common code instead of asking each derived class
overriding MSWGetBgBrushForChild() to do this.

This doesn't change anything but will make the upcoming changes to wxPanel
background painting simpler.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67250 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775