Improve drawing efficency by refactoring code

Improve drawing efficency by refactoring code

	* ../src/pgtkterm.c (fill_background, fill_background_by_face)
	(x_draw_glyph_string_background, x_draw_glyph_string_bg_rect)
	(x_draw_image_glyph_string, x_draw_stretch_glyph_string)
	(pgtk_clear_under_internal_border): Refator duplcate code

更に効率化。