support MJPEG based webcams

[Ioannis Nousias <s0238762 sms ed ac uk>]

Hi

I have a webcam that produces MJPEG video output. It's supported by the 
linux-uvc driver (v4l2).

Using my webcam with Cheese, displays a blank window. I presume the 
constructed pipe doesn't decode the MJPEG output to raw rgb.

Is it possible to add checks for the video format and automatically 
insert appropriate elements in the video pipe? For instance my camera 
works by inserting a 'decodebin' element. e.g.
gst-launch v4l2src ! decodebin ! xvimagesink

thank you for this nice application.

-Ioannis