Only use wxBU_EXACTFIT for width calculations in wxMSW wxButton.

wxBU_EXACTFIT should affect the width of the button best size but not its
height which should be at least the same as the height of a standard button
even when wxBU_EXACTFIT is specified, otherwise buttons created with it (like
the one in generic wxCollapsiblePane implementation) look completely ugly.

This commit restores the old behaviour which was recently changed by wxButton
sizing code simplifications.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67045 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775