Fix segmentation faults when subpass has unused attachments

[doitsujin]

This fixes two issues related to color attachments in subpasses.

1. Serialise_vkCmdClearAttachments uses the render pass attachment index from the subpass description without checking whether the attachment is used in the subpass.

2. VulkanDebugManager::RenderOverlay uses the wrong attachment indices for vkCmdClearAttachments. The correct index is the index into the color attachment reference array of the current subpass description, not the index into the attachment description array of the render pass description.
   Section 17.2 from the Vulkan specification says:

colorAttachment is only meaningful if VK_IMAGE_ASPECT_COLOR_BIT is set in aspectMask, in which
case it is an index to the pColorAttachments array in the VkSubpassDescription structure of the
current subpass which selects the color attachment to clear.

Issue #2 occurs when trying to use the "Clear before draw/pass" overlays in case the attachment index in the render pass description differs from the attachment index in the subpass description.

[doitsujin]

nevermind, wrong branch.