Fixes issues with readback of storage buffers modified on GPU for D3D
and Metal. Adds unit test for storage buffer readback.
D3D
* Fixes issue where QRhiBufferReadbackResult::completed callback could
be called twice on buffer readback completion.
Metal
* Fixes issue where buffer readback occurred prior to command buffer
being committed.
Change-Id: If55ac005f4438d66d2f65ea2e1ee0d5686c884ff
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>