WIP: add high-precision GPU trilinear interpolation for 3D LUTs

[cessen]

This addresses #1763.

The new high-precision code path can be enabled by disabling the new default-enabled OPTIMIZATION_NATIVE_GPU_TRILINEAR optimization flag.

The existing code path used the GPU's native trilinear texture interpolation function, which (although faster) quantized the lookup coordinates and could cause color banding. That's still the default, but now full-precision trilinear interpolation can optionally be used instead.

WIP

This is PR is WIP. It functions, but it's not ready to merge yet:

• It wasn't clear how best to get the optimization flags to the shader generation code in GetLut3DGPUShaderProgram(). I tried several approaches, but every approach ended up affecting some public API.
• The approach I finally landed on was to store the optimization flags in GPUProcessor::Impl, and then pass them to Op::extractGpuShaderInfo() via a new argument with a default value. Of all the approaches I tried, this seemed to have the lowest impact on both the code and the API.
• Unfortunately, this still breaks ABI compatibility (I think--I'm not totally sure how default arguments work at the ABI level in C++). Additionally, default arguments are apparently forbidden in the OCIO code base, so that's probably not the way forward, regardless.

I am very much open to suggestions for a better approach to get the optimization flags to GetLut3DGPUShaderProgram().

Additionally, I still need to add unit tests for the new code path.

Performance

Some initial naive performance tests indicate that the high precision code is notably slower than than GPU-native trilinear interpolation, but about on par with OCIO's tetrahedral interpolation. More testing is needed, however. For example, using higher-res LUTs and testing on a variety of GPUs. I'll update with actual data once I've had a better go at this.

[linux-foundation-easycla]

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• ✅ login: cessen / name: Nathan Vegdahl (2fbaf75)