We may be using unoptimised shaders

[AnyOldName3]

As a tangent to investigating something else, I realised that we're compiling shaders with glslang::SpvOptions::disableOptimizer set to true. That sounds bad, but at least historically, it doesn't seem to have been intended to optimise shaders properly anyway, and instead was just a badly-named variable controlling whether HLSL legalisation occurred when compiling a shader written in one language version to bytecode for a different shader model version.

The expectation, at least in 2019, was that the generated SPIR-V would be run through SPIRV-Tools-opt. I haven't been able to find anything confirming that that's changed.

If it is indeed still necessary, then it should be a fairly minor change to make, as we've already got a dependency on SPIRV-Tools via glslang.