WIP: overlayrenderer: implementation of overlay renderer that consumes new GstOverlayMeta meta data

A new element overlayrenderer, based on textoverlay, can render multiple text and rectangle shapes into the video frame overlay.

Here is a sample pipeline which displays overlay output of new onnxobjectdetector element:

uridecodebin  uri=file:///crowd_people_in_city_town.mp4  !
videoconvert !
onnxobjectdetector 
box-node-index=0
class-node-index=1
score-node-index=2
detection-node-index=3
execution-provider=cpu
model-file=/onnx-runtime-examples/data/models/model.onnx  label-file=/onnx-runtime-examples/data/labels/COCO_classes.txt  ! 
onnxoverlay !
overlayrenderer !
autovideoconvert !
autovideosink