Made exposures work. A few notes:
1. We're not doing something quite right in the calculation of the
   CCD's window because there are 2 bands of black pixels on both
   the left and the right side of the image (in a DARK frame).
2. The USB transfer mechanism as of right now is poor at best.
   All the io code is synchronous, happening from within the
   timer context -- bad, bad, bad! Asynchronous io is the key.
   A few things need to happen to make this work:
   a. Add support for write and error events to eventloop.c/h
   b. Add support for those new events through the indi API
   c. Use the new API with libusb_get_pollfds() to get async usb io