Change interpretation of font height in wxMSW to mean character height.

Accept both positive and negative height values in wxFont::SetPixelSize() in
wxMSW and map them both to a negative height when passing to MSW API in order
to request mapping against the character height and not the total cell height.

For positive heights this is more consistent with the other ports and also
expectations of people using this function. We keep the possibility to use
the negative heights inly for compatibility with the existing code which
worked around the (incorrect) interpretation of the positive height as cell
heights in the previous wxMSW versions by passing a negative height
explicitly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62566 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775