R16_SNORM, R16_UNORM, and R16G16_SNORM are required for computing cube
corner pixels for decompressed EAC formats.

Bug: b/138944025
Change-Id: I35a4d448e04bba7abc0e34250d153affe7fb1db5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34829
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>