Capability to get YUV image from VideoCapture object (Feature #1445)

The VideoCapture::Retrieve method populates a Matrix with an image capture by a camera. Currently, this method only supports retrieving the image in BGR, RGB, and GRAY formats. It is proposed to add the YUV format to this list.

The reasoning for this is:

In Android devices, the image captured by the camera is in YUV format. In order to get these other formats, a conversion must be performed performed. This conversion is unavoidable in nearly all cases because YUV isn't a particularly useful format, but there is one case in which it is not desired.

Should the user require both a GRAY and RGBA format image of the same frame, the only option is to capture the RGBA format image and convert it to GRAY. However, the RGBA->GRAY conversion is much slower than the YUV->GRAY conversion. If the user had the option to capture the YUV, then they could do the YUV->RGBA, and YUV->GRAY conversions directly. The performance enhancement would be on the order of 10 milliseconds. This may seem small, but in an application running at 30 FPS, this a significant portion of the time available for each frame.