Device-specific initialisation time for Blackmagic cards

[dzhoshkun]

This is specifically related to the DeckLink SDI 4K. The current VideoSourceBlackmagicSDK constructor includes an artificial thread sleep to allow for the device to start streaming. This is sufficient for the DeckLink 4K Extreme 12G to have 100 % success rate when repeatedly running the leak checker script (see #43). However with the SDI 4K the success rate is only about 90 %, unless the artificial sleep is increased to 250 msec.

As this initialisation time seems to be coupled to the actual Blackmagic hardware (as similar SDK versions of 10.11.2 and 10.11.1 are used for the Extreme and SDI 4K respectively), it might be a good idea to factor it out to the factory via an optional parameter to VideoSourceBlackmagicSDK instead of using a generic value for all Blackmagic devices.