Add Stipple support for PGTK

Add Stipple support for PGTK

* ../src/pgtkterm.c (create_background_surface_by_face)
(create_background_surface, x_draw_glyph_string_background)
(x_draw_glyph_string_bg_rect, x_draw_image_glyph_string)
(x_draw_stretch_glyph_string, pgtk_draw_fringe_bitmap): handle stipple

* ../src/image.c (image_create_bitmap_from_file, )
(syms_of_image): Add pgtk support

* ../lisp/faces.el (face-valid-attribute-values): add pgtk support

stipple 対応。

stipple がまともに動いてなかったので修正。