Added an optimization that fixes a recursive paint problem when controls

Added an optimization that fixes a recursive paint problem when controls
are embedded in wxHTML (or other circumstances where the parent resizes
the choice/combobox within a paint handler). This also speeds up
addition of strings considerably (see "Add many strings" in widgets sample).

Also added a check for silly values that can be returned from
CB_GETDROPPEDCONTROLRECT.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38301 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775