* Enumerate frame sizes once rather than twice.
 * Do not enumerate frame rates if not supported.
 * Get actual frame rate from the device driverr.
 * Get exact fractional frame rate rather than round to single precision
   floating point.

Notes:
 * --v4l2-fps becomes totally redumdant. It should probably be redefined
   to select a maximum capture frame rate.
 * --v4l2-width and --v4l2-height are ignored. This is a regression.
   Maybe they should be redefined as maxima as well as --v4l2-fps.