Extend qrhi autotest exercising APIs with offscreen targets

Some functionality, even though not directly rendering-related, cannot be tested without preparing a frame. Use only beginOffscreenFrame() in this task, meaning no swapchain and QWindow needs to be created. If a renderpass is needed use a texture as the target.

Leave testing full-blown onscreen cases to QTBUG-78972.

Things to exercise:

• nativeHandles() on various objects
• importing context/device/texture
• QRhiResourceUpdateBatch. creating, submitting, merging.
  • buffer operations (update/upload)
  • texture operations (upload, copy, readback)
• beginOffscreenFrame, clearing and rendering something simple to a texture