This is the implementation for vkCmdClearAttachments.

It includes the new command along with the plumbing to
get the information to the Image clear function.

The only non trivial behavior here is that the provided
baseArrayLayer for the clear command is to be offset by
the ImageView's subresourceRange's baseArrayLayer so
that it clears the correct layers.

Bug b/119621736

Passes all tests in:
api.image_clearing.dedicated_allocation.clear_color_attachment.*

Change-Id: I19f86b63239ca2fb4d53f2c6c9cb75185794f061
Reviewed-on: https://swiftshader-review.googlesource.com/c/23748Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>