Autodiscover

[timmarkhuff]

• [x] Added some logic to FrameGrabber.autodiscover and FrameGrabber.create_grabbers so that they are sorted by serial_number (if there are serial numbers) before they are returned to the user. This way, the user is guaranteed that the grabbers always come in the same order and with the same names.
• [x] Added a “num_90_deg_rotations” option so that the user can rotate frames. This is needed for the URCap.
• [x] Fixed a buffering issue in GenericUSBFrameGrabber.grab that was causing a stale frame to be returned