Using wxRendererNative::GetCheckBoxSize() as the size of wxCheckBox just
doesn't work with GTK 3, as the control has additional padding between
its actual contents and the focus rectangle, which means that its actual
size must be greater than the size to be passed to DrawCheckBox() in
order to draw a checkbox of the same size.
However it isn't really necessary to resize wxCheckBox at all, it's
enough for DrawCheckBox() to produce a check mark of the same size as
that shown in a default-sized wxCheckBox and this does work in wxGTK.
So keep the default size of wxCheckBox to make everything work.
This means wxGetGridCheckBoxRect() is not useful any more, so replace it
with wxGetContentRect() which just positions a rectangle of the given
size inside another one (this should probably be moved somewhere else,
as it's more general than wxGrid).
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