(x_set_frame_parameters): Avoid infinite recursion for

(x_set_frame_parameters): Avoid infinite recursion for
some items.
(x_set_internal_border_width): Set frame garbaged when window
doesn't exist yet.
(Fx_create_frame): Accept zero tool_bar_button_relief.
(x_clear_image_1, four_corners_best, image_background)
(image_background_transparent): New functions.
(xpm_format, png_format, jpeg_format, tiff_format, gif_format)
(gs_format): Add `:background' entry.
(lookup_image): Set IMG's background color if specified.
(pbm_load, xbm_load_image, png_load): Set IMG's background field
when appropriate.
(x_clear_image_1): Reset `background_valid' and
`background_transparent_valid' fields.
(x_build_heuristic_mask): Use IMAGE_BACKGROUND instead of
calculating it here.  Set IMG's background_transparent field.
(enum xpm_keyword_index): Add XPM_BACKGROUND.
(enum png_keyword_index): Add PNG_BACKGROUND.
(enum jpeg_keyword_index): Add JPEG_BACKGROUND.
(enum tiff_keyword_index): Add TIFF_BACKGROUND.
(enum gif_keyword_index): Add GIF_BACKGROUND.
(enum gs_keyword_index): Add GS_BACKGROUND.
(pbm_load, png_load, jpeg_load, tiff_load, gif_load):
Pre-calculate image background color where necessary.
(x_create_x_image_and_pixmap, xbm_load, gs_load): Use
display info's n_cbits entry for screen depth.
(Fx_show_tip): Remove unused variables `buffer', `top',
`left', `max_width' and `max_height'.