(x_alloc_nearest_color_1): New function.

(x_alloc_nearest_color_1): New function.
(x_alloc_nearest_color): Use it.
(x_color_cells): Take a Display as argument, instead of a frame.
(string_to_pixel_args, cvt_string_to_pixel_value): New variables.
(cvt_string_to_pixel, cvt_pixel_dtor): New functions.
(x_initialize): Register cvt_string_to_pixel as resource converter
string -> Pixel and cvt_pixel_dtor as pixel resource destructor.