[Reg 5.15 -> 6.x] QSGGeometry::DrawLineLoop is no longer supported

https://doc.qt.io/qt-6/qsggeometry.html#DrawingMode-enum lists QSGGeometry::DrawLineLoop (== GL_LINE_LOOP == 0x02) as a drawing mode, but it doesn't work in Qt 6

Steps to reproduce

1. Build and run the graphs example https://github.com/qt/qtdeclarative/blob/v6.5.0-beta2/examples/quick/scenegraph/graph
2. Modify linenode.cpp and replace QSGGeometry::DrawTriangleStrip with QSGGeometry::DrawLineLoop
3. Build and run the example again

Outcomes

• Qt 5.15 applies the new mode, as expected
• Qt 6 applies the default value, QSGGeometry::DrawTriangles, and produces a warning:

Primitive topology 0x2 not supported

Other notes

Customer reported that GL_POLYGON == 0x09 also worked in Qt 5 but not in Qt 6