/** Create an image for this frame. A lock must be held on _mutex.
* @param pixel_format Function which is called to decide what pixel format the output image should be;
* it is passed the pixel format of the input image from the ImageProxy, and should return the desired
* output pixel format. Two functions force and keep_xyz_or_rgb are provided for use here.
/** Create an image for this frame. A lock must be held on _mutex.
* @param pixel_format Function which is called to decide what pixel format the output image should be;
* it is passed the pixel format of the input image from the ImageProxy, and should return the desired
* output pixel format. Two functions force and keep_xyz_or_rgb are provided for use here.