GLSL Struct initialization complicates MSL cross compilation

While looking at getting Masuo's Hextiling PR working #2094 for MSL, I discovered that GLSL and MSL differ in how structs are initialized.

I think its an easy refactor for the Hextiling PR, but noting this difference here, so we can investigate a more robust solution in the future.

Below is the problematic code from Masuo's PR.
```
HextileData tile_data = HextileData(
st1, st2, st3,
vec3(w1, w2, w3),
trm1, trm2, trm3,
ddx1, ddx2, ddx3,
ddy1, ddy2, ddy3
);
```

The workaround is to create the object and then initialize each member separately.

```
HextileData tile_data;

tile_data.coord1 = st1;
...
```

But I can imagine cases where this might not be desirable.

Start by reviewing the Hextiling PR #2094 and the problematic HextileData initialization shown here, then compare the GLSL and MSL struct-initialization behavior. Done means agreeing on and implementing a robust cross-compilation approach that avoids the current member-by-member workaround.

Written by the indexing model from the issue text.