- added many bad BMP files to test various code paths in bmp.c, and

- added many bad BMP files to test various code paths in bmp.c, and
  made many minor fixes to bmp.c to make them work:
  - it was possible for various types of read failures to SEGV, both
    as NULL pointer dereferences and buffer overflows
  - some errors, like palettes being too large for the number of bits
    per pixel, were not being caught
  - failing to read all of a packed data structure would not cause
    a read failure
  - invalid compression types were not always caught
  - error messages are more consistent (still not always great messages,
    but one step at a time)
- added bmp_compression_name, bmp_used_colors, bmp_filesize, bmp_bit_count
  tags on reading a BMP file