fakeFrameGrabber enhancements

[drdanz]

Devices

fakeFrameGrabber

• The getImage() method no longer blocks the caller.
  The --syncro option was added to restore the old behavior.

• The topIsLow option and the set_topIsLow rpc command were added to produce
  images with bottom to top scanlines.

• The nois mode (introduced in fakeFrameGrabber_enh) was removed in favour of
  the --noise option and set_noise and set_snr commands that adds noise
  to the generated images.

Depends on #2593 and #2583