The attachment index in the `VkClearAttachment` struct is an index into
the current subpass attachment array, not the render pass attachment
array; so it's not enough to check `VkClearAttachment` for
`VK_ATTACHMENT_UNUSED`. We also need to check the current subpass.
If no attachments can be cleared, don't encode a command to the Metal
command buffer.
Perhaps I should bring back `recordBeginRenderPass()` and
`recordEndRenderPass()` and keep track of the current subpass; then we
can avoid recording the command entirely if the intersection of the sets
"attachments to clear" and "used attachments" is null.
2d7717b708