X11 XReadBitmap API support.

G'day,

This patch adds support for the libX11 XReadBitmapFile API. This resolve a downstream support use case issue for XmoBar github issue #87.

Background found here:
https://github.com/jaor/xmobar/pull/87

May we please merge this here upstream for other possible uses, perhaps in XMonad for example.

Kind Regards,
Edward O'Callaghan.