Portland Bitmap Font format

This format is intended as a quickly readable font format.

Header:
  byte: character width
  byte: character height
  byte: horizontal padding
  byte: vertical padding
  dword: offset into file of default character bitmap

For each character (255 of them, 0x01 to 0xff):
  dword: offset into bitmap data area of character bitmap,
           or 0xffffffff if the default should be used.

Bitmap data area:
  bitmaps are from left to right, then from top to bottom.
  the least significant bit of a byte is the "first" one.