Accessors should accept runtime-sized schemas

[iwoplaza]

const ImageStruct = (count: number) =>
  d.struct({
    width: d.u32,
    height: d.u32,
    pixels: d.arrayOf(d.vec4f, count),
  });

// Before
const imageAccess = tgpu.accessor(ImageStruct(0)); // hack to instantiate runtime-sized schema
// After
const imageAccess = tgpu.accessor(ImageStruct);