Depth buffer in a multiview render pass

QRhiRenderBuffer does not work anymore. Must use a texture array for depth/stencil as well and use setDepthTexture() on the render target.

This means OpenGL ES 3.0 is probably out too, although that's not a big loss in the context of multiview.

Exercise this in the manual test. Some backends may need some fixes.

Only covers depth. Turns out stencil is complicated (at least with OpenGL, and Metal may need some fixes too for that)