Handle specified bg in images. Use generic libpng code for PNG:s.

Handle specified bg in images.  Use generic libpng code for PNG:s.

* src/image.c (get_spec_bg_or_alpha_as_argb)
(create_cairo_image_surface): New functions when USE_CAIRO.
(xpm_load): Call the above functions.  Handle XPM without mask
when USE_CAIRO.
(png_load_body): Handle USE_CAIRO case.
(png_load): Remove USE_CAIRO specific fuction, modify png_load_body
instead.
(jpeg_load_body): Call create_cairo_image_surface.
(gif_load, svg_load_image): Handle specified background, call
create_cairo_image_surface.

* src/xterm.c (x_draw_image_glyph_string): Added missing USE_CAIRO.