The fixed-function OpenGL ES 1.1 and Direct3D 8 texture sampling
operations used 4.12 fixed-point values. SPIR-V only returns float4
or int4 results.

Also eliminate handling of 'null' textures. Vulkan 1.1 demands valid
descriptors

Bug b/129523279

Change-Id: I692bcaced55082b41a9c8a18551c23b03cb18430
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29272
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>