Cleanup the font driver a successful font load.

By default, a font driver is cleaned up either after an unsuccessful font load or on the next attempt to load a font with this driver; any open files or allocated memory are cleaned when exiting the program, but I'm not sure about the X11 and Windows resources. Anyway, alwaus cleaning up after a font load seems more proper to me than to wait for the next load or rely on the C library cleanup at exit.

I tracked down the calls to the driver functions and tested the patch with GrLoadFont() and GrBuildConvertedFont(). It seems to work, and the input font file can now be open for writing, unlinked and so on, but more testing is required.

E-gards: Jimmy