Allow using :width/:height as normal with xbm images

Allow using :width/:height as normal with xbm images

* doc/lispref/display.texi (XBM Images): Adjust the documentation.
* src/image.c (enum xbm_keyword_index): Add :data-width and
:data-height.
(xbm_format): Ditto.
(xbm_image_p): Allow passing in :width/:height for display.
(xbm_load): Use :data-width/:data-height.