Code generation only works for PixelShader (Dx11) / Fragment Shader (OpenGL)

When performing lookup table sampling, OpenColorIO uses the following :

`outColor.r = ocio_lut1d_0.Sample(ocio_lut1d_0Sampler, ocio_lut1d_0_computePos(outColor.r)).r;`

This makes it impossible to use the generated code outside of a pixel shader, since in compute or other stages, we are not able to have automatic mip selection.

generated code should be :

`outColor.r = ocio_lut1d_0.SampleLevel(ocio_lut1d_0Sampler, ocio_lut1d_0_computePos(outColor.r),0).r;`

This is the same in OpenGL where we should enforce LOD = 0

This change should be for both 3d and 1d lookup textures.

I'm happy to make a PR if needed, if you prefer to avoid potential breaking changes, I'm also happy to make it as an Option in settings:

`shaderDesc->useLodSampling(true);

Thank
Julien