Add support for X11 XReadBitmapFile API. This resolve a downstream support use case issue for XmoBar github issue #87.
Properly handle XReadBitmapFile return values.

Signed-off-by: Edward O'Callaghan <victoredwardocallaghan@gmail.com>
Merge pull request #19 from victoredwardocallaghan/master

X11 XReadBitmap API support.