Added new method getPad to be able to set properties on pads

[chfritz]

Example:

const pipeline = new gstreamer.Pipeline('videotestsrc pattern=1 ! video/x-raw,format=AYUV,framerate=\(fraction\)30/1,width=100,height=100 !    videobox border-alpha=0 top=-70 bottom=-70 right=-220 ! compositor name=comp sink_0::alpha=0.7 sink_1::xpos=600 sink_2::xpos=200 !    videoconvert ! xvimagesink    videotestsrc !    video/x-raw,format=AYUV,framerate=\(fraction\)5/1,width=320,height=240 ! comp. autovideosrc ! video/x-raw,framerate=\(fraction\)30/1,width=320,height=240 ! comp.')

pipeline.play()
const pad = pipeline.getPad('comp', 'sink_2')
pad.xpos = 300; // dynamically move the image within the composition
pad.alpha = 0.5;  // dynamically change the transparency of the sub-image

This starts a compositor pipeline with a webcam and two test streams, then uses the gotten pad to change properties of one of the subimages dynamically.

[dturing]

Thanks @chfritz