Fix assert message when removing non-existent items from wxMenu.

The assert complained about a bug in wxMenu code itself, but actually could be
triggered by simply trying to remove a non-existent item from the menu.

Fix this by checking for the item existence, and also removing it from the
internal menu data structure, in wxMenuBase::Remove() and only dealing with
the item itself in DoRemove().

Closes #3424.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75250 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix wxRegion::Offset() copy on write behaviour in wxMSW.

Offset the correct, new and unshared, region handle instead of the old and
possibly shared one.

Closes #15690.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75251 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775