Underlying nvoverlaysink graphics engine - retrieve resoltion

Hi,
The nvcompositor plugin decides the resolution per input sources. For example, if you have two 640x480 sources and put them syde-by-side, the output resolution is 1280x480.

If you would like to set up the resolution, may apply this patch to add the property:
Gst for merging 4 webcams sometimes works, and sometimes it doesn't - #5 by DaneLLL