Implement SNORM uniform texel buffer support
Implement SNORM uniform texel buffer support

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 <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Reviewed-by: Antonio Maiorano <86d87a6550afb6b07035c9743fa181900589f7e0@google.com>
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader