Add options for Hlsl shader generator to allow register selection.

Hello,

in the current state OCIO does not allow generated code to manually specify register slots (d3d11/12):

Example of generated code :

`Texture2D ocio_lut1d_0;
SamplerState ocio_lut1d_0Sampler;`

This makes reflecting compiled shader (Nearly) mandatory to find the binding location, which is not desirable in many cases.

It would be nice to have an option to be able to set first register slot by hand, then get them generated in a fixed way, for example :

`shaderDesc->setLanguage(GPU_LANGUAGE_HLSL_DX11);
shaderDesc->setFunctionName(functionName);
shaderDesc->setResourceStartSlot(1);
shaderDesc->setSamplerStartSlot(4);

Generated code would become:

`Texture2D ocio_lut1d_0 : register(t1);
SamplerState ocio_lut1d_0Sampler : register(s4) `

On a later stage, supporting bindless for latest d3d12/vulkan implementations would also be nice.

Thanks
Julien