[Frame loop] Copy vkbuffer using shaders

Currently, the back/restore for vkbuffer is done by `vkCmdCopyBuffer`, which assumes that the source buffer has the `VK_BUFFER_USAGE_TRANSFER_SRC_BIT` bit set during creation. The validation layer will complain this if the source buffer is not created with that bit set. So need to implement the buffer copy via the share, which should be similar (and simpler) to image primer.