Correct creation of the mask for wxImage cursors in wxGTK.

The code created the monochrome bitmap used by wxCursor(wxImage) ctor
incorrectly resulting in bad cursor appearance. Use the right values for
foreground and background pixels (which are inversed compared to naive
expectations) to fix this.

Closes #11989.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65107 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775