Also add stubs for Dref variants.

Bug: b/129523279
Test: dEQP-VK.glsl.texture_functions.*
Test: dEQP-VK.spirv_assembly.instruction.graphics.image_sampler.*
Change-Id: I810c8e32758c9124f7649c62d51b34751ee9bfae
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30193
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>