Allow splitter component to process still images about mmalsharp

[techyian]

This appears to be working well, however if the user intends on outputting via image encoders then they must add a Resize/ISP component between the splitter output port and the image encoder input port. If the user doesn't do this, the splitter component only seems to allow 1 encoder component to be connected to any given splitter output port. Raw capture from the splitter works well too.

The default constructor for a splitter component will instantiate 4 SplitterVideoPort objects to be used for its output ports which are intended for video recording functionality. The user can override this when configuring the output port by using SplitterStillPort as a generic constraint.

[techyian]

I've added some documentation around this which should hopefully be clear on how to use the splitter component with the camera's still port.