Prove that the BSDF prefiltering compute pass is implementable with QRhi

On GLES 3.1+, GL 4.3+, and always with other QRhi backends, we can use compute shaders. So Quick3D's compute shader based HDR filtering can be ported with little effort, in theory. Prove that that is the case.

This also serves as an opportunity to add a manual test that exercises RGBA16F, also in combination with compute.

Update: Turns out this was a pretty good move since it uncovers a problem of lack of barriers between compute dispatches. To be seen how that gets solved. Handled in QTBUG-81217.