QBufferCapture object

Compute shaders are ussually used to do some heavy and parallel computations on the GPU side. The result from this stage is not always used in GPU for drawing but on CPU side for other purposes (eg, an animation system with performs vertex transformation on GPU and then must save this transformed geometry to a file).

It would be nice to have a way to transfer the buffer data from GPU to CPU side.

I believe this can be coded in a similar way as the QRenderCapture class (therefore the QBufferCapture name).

If you agree with this, I can coded and try to have it ready before the 5.9 freeze feature deadline.