D16_UNORM is the first format supported in this path with a texel size
of less than 4 bytes. Adjust the loading to round up the size so we always
emit at least one load.

Test: dEQP-VK.renderpass*
Bug: b/131171141
Change-Id: I985b85017ace84cb8b18c36989fea99ba8e0f44f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29729
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>