OpenGL backend produces flipped rendering in some cases

Certain lancelot scenes produce different result with OpenGL compared to Metal/Vulkan/D3D, because of Y-Up difference.

1. custommaterial/lightmapgen.qml

1. textures/view3d_as_texture_in_view3d.qml
   textures/view3d_as_texture_in_view3d_change.qml

1. effects/aberration.qml
   effects/brushstrokes.qml
   effects/colors.qml
   effects/custom_effect_simple.qml
   effects/distortion1.qml
   effects/distortion2.qml
   effects/edge.qml
   effects/sequence.qml

See e.g. http://lancelot.intra.qt.io/BaselineComparison/2021-03-22_4/

For the effects, there seem to be two closely related issues at play. One is the y-up orientation of buffers/textures. Another is that some effects take a parameter that is a coordinate on the rendered image - where the Y axis points to opposite way. For example, see the top right field of effects/distortion2.qml. The rotation center is set to (0.4, 0.8). That ends up in the top half of the image under OpenGL, and the bottom half under Metal/Vulkan/D3D.