WebGL plugin does not work since RHI landed in qtbase and qtdeclarative

Qt WebGL QPA plugin stopped working since RHI support landed, specifically the following commit. https://codereview.qt-project.org/c/qt/qtdeclarative/+/259447

How to reproduce:
1) set environment variable QT_QPA_PLATFORM=webgl
2) Start qtbase/bin/qmlscene qt_source/qtwebglplugin/tests/plugins/platforms/webgl/basic_scene.qml
3) Open browser supporting webgl and point to 127.0.0.1:8080 (I used Chrome)
4) Observe endless loading spinner animation.

More details in the WIP commit I will push