Don't draw over dummy separators in MSW toolbar, just don't draw them at all.

Don't draw over dummy separators in MSW toolbar, just don't draw them at all.

Prevent the native toolbar from drawing the separators which we use simply as
placeholders by excluding them from the update region when handling WM_PAINT.

This reduces flicker by not redrawing the areas occupied by these separators
twice and also prevents them from ever being shown (you could see them briefly
appear before being erased before).

The other toolbar tools still flicker though, there doesn't seem to be any
simple way to prevent the control from entirely invalidating itself whenever
it is resized.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62934 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775