Fix sizing issue in GTK+ widget.
Fix sizing issue in GTK+ widget.

Rather than use an unusual default size of 600 x SCREEN_HEIGHT,
request -1 x -1 for the natural widget size.

rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror,rhaberkorn/scintilla-mirror