anv/meta: Rewrite clear code

anv/meta: Rewrite clear code

Fixes Crucible test "func.clear.load-clear.attachments-8".

The old clear code, when clearing attachments for
VK_ATTACHMENT_LOAD_OP_CLEAR, suffered from some fundamental bugs. The
bugs were not fixable with the old code's approach.

    - It assumed that a VkRenderPass contained at most one depthstencil
       attachment.

    - It tried to clear all attachments (color and the sole
      depthstencil) with a single instanced draw call, using the VUE
      header's RenderTargetArrayIndex to specify the instance's target
      color attachment. But the RenderTargetArrayIndex does not select
      entries in the binding table; it only selects an array index of
      a singled layered surface.

    - If at least one attachment of VkRenderPass had
      VK_ATTACHMENT_LOAD_OP_CLEAR,
      then the old code cleared *all* attachments. This was
      a consequence of using a single draw call and single pipeline for
      the clear.

The new clear code fixes those bugs by making a separate draw call for
each attachment, and using one pipeline when clearing color attachments
and a different pipeline for depth attachments.

The new code, like the old code, does not clear stencil attachments. It
is left as a FINISHME.