* io-tga.c (get_contiguous_pixbuf): Helper function to create
	a pixbuf with a contiguous pixel array while being careful about
	overflow.

	* io-tga.c (fill_in_context, get_image_pseudocolor,
	get_image_truecolor, get_image_grayscale): Use
	get_contiguous_pixbuf instead of manually allocating image
	storage.

	* io-xpm.c (pixbuf_create_from_xpm):
	* io-pnm.c (gdk_pixbuf__pnm_image_load):
	* io-jpeg.c (gdk_pixbuf__jpeg_image_load): Use gdk_pixbuf_new
	instead of manually allocating image storage.