qmlgloverlay: fix multiple elements with Qt 5.15
With Qt 5.15 multiple qmlgloverlay elements would produce:
ASSERT: "!m_gl->property(QSG_RENDERCONTEXT_PROPERTY).isValid()" in file /path/to/qt5/qtdeclarative/src/quick/scenegraph/qsgdefaultrendercontext.cpp, line 121
Workaround by setting the (seeminigly unused) property before initialization.