avfvideosrc should implement Gstphotography interface

Submitted by Shahab

Link to original bug (#756945)

Description

We would expect avfvideosrc would implement the GstPhotography interface for properties like (Focus mode, Exposure mode, White balance mode)

There are two Exposure modes:

AVCaptureExposureModeContinuousAutoExposure and
AVCaptureExposureModeLocked

There are three focus modes:

AVCaptureFocusModeLocked
AVCaptureFocusModeAutoFocus
AVCaptureFocusModeContinuousAutoFocus

There are two white balance modes:

AVCaptureWhiteBalanceModeLocked
AVCaptureWhiteBalanceModeContinuousAutoWhiteBalance

It would be great if we could set the enum values in respected modes. Till now we can not set the modes in gst-plugins-bad/edit/master/sys/applemedia/avfvideosrc.m file

Thank you.

Version: 1.6.0