Reads a Lisp Machine bitmap as input. Produces a portable graymap as output.
This is the file format written by the tv:write-bit-array-file function on TI Explorer and Symbolics lisp machines.
The Lispm bitmap file format is a bit quirky; Usually the image in the file has its width rounded up to the next higher multiple of 32, but not always. If the width is not a multiple of 32, we don't deal with it properly, but because of the Lispm microcode, such arrays are probably not image data anyway.
Also, the lispm code for saving bitmaps has a bug, in that if you are writing a bitmap which is not mod32 across, the file may be up to 7 bits too short! They round down instead of up, and we don't handle this bug gracefully.