Determine the offset/digital gain for imx296 and imx462 sensors

[brickbots]

In testing the imx477 there is an offset to the black level in the raw output, and that star detection/solve robustness was improved by removing this offset and adding a digital gain prior to converting to an 8 bit image. This offset/gain is available for all sensors, but only have experimentally validated values for the imx477.

See #335 for details

This issue is to perform simulated and real-world tests to see if the other sensors have a black level offset and to determine what, if any, digital gain will help them perform their best.