VK_FORMAT_R8_SNORM, R8G8_SNORM, and A8B8G8R8_SNORM_PACK32 are mandatory
uniform texel buffer formats, which we claimed support for, but their
implementation was missing.

Bug: b/159561781
Change-Id: Ia66d14cd8189a664bf41af7d918260d2524aa63a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46009
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>