Allow a bg_pixmap of "<none>", as there was no way to set the background

Allow a bg_pixmap of "<none>", as there was no way to set the background

Sun Jan 17 12:01:57 1999  Owen Taylor  <otaylor@gtk.org>

	* gtk/gtkrc.c (gtk_rc_style_init): Allow a bg_pixmap
	of "<none>", as there was no way to set the background
	on all widget but some widgets. This is important
	because Text widgets with background images are
	slow and possibly ugly.