be sure to store images in this bitmap upside down! this reader is
expecting mips stored like so (after flipping):

   +--------+.........
   |        |        .
   |   1    +---+    .
   |        | 2 +-+  .
   |        |   |3+-+.
   +--------+---+-+-++
   |                 |
   |                 |
   |                 |
   |                 |
   |        0        |
   |                 |
   |                 |
   |                 |
   +-----------------+

if there are no hand-made mipmaps then it will construct them on top of
the 0 surface. each successive request for a new surface will box filter
the surface down over itself.