v4l2: adding cropping and compose functionality

Submitted by Dimitrios Katsaros

Description

This patch adds cropping and composition to the v4l2src. The reason for limiting the patch to the source is the lack of a v4l2 output device to test on. The device available also only supports cropping, so composition also requires additional testing

This patch has been developed with the current master branch, commit id: 8cfebfec