- Before, we supposed that the maximum number of input attachment
   should match the maximum number of color attachments. But in
   reality, we've only used one input attachment for the second
   subpass.
 - The problem with the above supposition is that the descriptor
   set layout for the input attachment descriptor set must have the
   exact number of input attachment specified in the shader. If the
   *layout* has more input attachment slots than specified in the
   shader, then we'd run into a validation error.
 - In this patch, we fix the number of max input attachment in
   the descriptor set layout to 1, since we ever only make use of
   one.

Fixes #6513