tricked xwin driver to not show brievely PIXMAP windows as it was doing before
removing antialiasing for wxWidget plot windows improves greatly the use of XOR or INVERT graphic functions used, e.g., in BOX_CURSOR. (besides, speeds up a lot the plots).
Antialiasing can be forced by creating an environment variable GDL_DO_ANTIALIASING.
Checked thorougly that with this version, all X11 or wxWidgets uses of BOX_CURSOR were OK under linux.