Treat raytracing::acceleration_structure as a texture resource in argument buffer

[shobomaru]

Currently, acceleration structures in argument buffers are treated as buffers, so the constant qualifier and a pointer are added to raytracing::acceleration_structure.
This results in code that does not compile.

struct spvDescriptorSetBuffer0
{
    constant raytracing::acceleration_structure<raytracing::instancing>* RTAS [[id(0)]];
};

This PR fixes the issue by handling acceleration structures in argument buffers in the same way as textures, ensuring that the generated code is compilable.

struct spvDescriptorSetBuffer0
{
    raytracing::acceleration_structure<raytracing::instancing> RTAS [[id(0)]];
};

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