Investigate lancelot discrepancies between RHI and direct OpenGL

We still have some rendering differences in Lancelot tests  between the old direct GL code path and the RHI path with the OpenGL backend. Some are spurious pixel differences, but others need to be handled. Either by fixing the issues, by creating bug reports to be fixed later, or by documenting an intended difference between QtQuick3D 5.15 and 6.0

 Visible differences:

• view3d/ClearModeSkyBox.qml – pixelated slightly differently [Barely visible: probably rounding]
• custommaterials/PaperArtistic.qml and others – directGL code sets number of mipmaps to the wrong value (QTBUG-83627 ) [FIXED]
• effects/brushstrokes.qml – offset/orientation different (QTBUG-83812)
• effects/depth.qml, misc.qml, move.qml – RHI has more blur [FIXED]
• effects/tonemaps.qml – HDRBloomTonemap looks different [FIXED]
• layers/Ambient_occlusion.qml – RHI has visible banding. Looks like DirectGL does not do SSAO on the first frame. Also, DirectGL only applies SSAO to opaque objects: QTBUG-83874
• layers/Light_probe.qml – RHI is missing some light (QTBUG-83627 ) [FIXED]
• lighting/Imagebasedlighting.qml – RHI is much darker (QTBUG-83627 ) [FIXED]
• principled/normal.qml – DirectGL renders objects completely black [FIXED]
• quick/DynamicCreation.qml, MultipleItems.qml – RHI renders nothing QTBUG-83469 [FIXED]
• quick/RenderingOrder.qml – RHI renders less than half the scene QTBUG-83469  [FIXED]
• quick/Visibility.qml – RHI renders nothing (text issue?) QTBUG-83469 [FIXED]
• view3d/RenderModeMultiple.qml – RHI misses clearColor. Text is stretched with RHI. QTBUG-83801 QTBUG-83816
• view3d/RenderModeUnderlay.qml – RHI misses clearColor QTBUG-83801

 Also: defaultmaterial/BlendModes.qml is very broken with directGL, so hard to say if correct with RHI.

**Note: brushstrokes.qml offset issue on RHI/GL seems to have been fixed