Split auto exposure and gain controller for WMR

It is a better idea to adjust exposure and gain individually for each camera to avoid situations like the ones in the example below. The right camera is getting overexposed by using the exposure and gain values from the left camera; thus making feature tracking difficult. This is now more obvious now that we can track features on the entire right camera image (mateosss/basalt!20 (merged)).

A similar MR should be done for the Rift S driver but I don't have such device at hand to test.