support frames with sprite_16x16.png style naming
support frames with sprite_16x16.png style naming

- every image now also includes width and height as the first 2 bytes
- this will tell the generator that you're intending to have multiple
  frames inside the image, each 16x16.
- your frames must be vertically oriented
- the generated .h file will have comments counting off each frame

yyyc514/img2ard