-------------------------X11 BITMAP (.xbm) IMAGE FORMAT-------------------------
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X11 bitmaps are black-and-white raster images stored as fragments of C source
code.  Each code fragment defines two preprocessor directives, ``IMAGE_width''
(which evaluates to the width of the image) and ``IMAGE_height'' (which
evaluates to the height of the image), and an array of unsigned chars (usually
declared with the ``static'' storage-class specifier) named ``IMAGE_bits'' in
which the contents of the image are stored (``IMAGE'' here represents the
basename of the image file).

If the image is meant for use as a pointer icon, two additional preprocessor
directives are defined: ``IMAGE_x_hot'' and ``IMAGE_y_hot'', which evaluate to
the x- and y-coordinates, respectively, of the pointer's "hot spot."  The
coordinate system used for these directives has the pixel in the upper-left
corner of the image as the origin, with the x-axis increasing to the right and
the y-axis increasing downwards.

Within ``IMAGE_bits'', each row of the image is represented by one or more
bytes in which each bit corresponds to a pixel from the row, from left to
right, stored in successive bytes from the least significant bit to the most
significant bit.  An unset bit represents white, and a set bit represents
black.  If the width of the image is not a multiple of eight, the remaining
bits in the last byte for each row are left unset.  Each row in an image is
represented by the same number of bytes, which is as small as possible, and the
rows are stored in the array contiguously from top to bottom.