write_xbm_image doesn't sanitize fileName argument

The write_xbm_image() function assumes that the filename is a C-compatible identifier:

qsnprintf(buf, msize, "#define %s_width %d\n", fileName.toUtf8().data(), image.width());

The file name seems to be set using options, so could be anything.

I don't know how the XBM format is defined, but at least a file name containing "meep_width 1000000\n#define ignoreme" looks like it will produce an ill-formed output file.