Terrain: optimize WebGPU compute shader by fixing patch size

[piellardj]

Now that the map is infinite, patch size should be constant everywhere, which could allow for some optimizations in the WebGPU compute shader.

Instead of apssing the cache size as a storage struct variable:

struct LocalMapCacheBuffer {
            size: vec3i,
            data: array<u32>,
        };

it becomes a compile-time constant, which could directly be injected into the shader.

[piellardj]

No longer needed since we now compute patches on CPU. Moreover, in my test this change did not improve performances.