WGSL will require this. Note that this still might
cause some issues with multi-entry-point GLSL
that I didn't know how to handle. That is, we will
handle unused builtin inputs but not unused
builtin outputs correctly right now. This is an existing
issue though, not a regression.

This is also provided as an option, but I feel like
the more correct approach is to never strip linkage
variables. We'll see though.

For debugging. Maybe there's a way to do it for all tests,
not sure...

* [wgsl-in] Map interpolate(..., center) to Sampling::Center

* [wgsl-out] Improve interpolation attributes

* [wgsl] Fix tests/in/interpolate.wgsl typo/format

* [wgsl] Snapshot WGSL artefacts

* [wgsl-out] Stack-only writing of interpolation

Glsl doesn't have depth textures what it has is combined image/samplerShadow

Since `BlockContext` now owns the `cached` table directly, there's no point in
going through a helper function.

Since the table of cached expressions is only meaningful within a single
function, it's really something that should only be accessed from
`BlockContext`.

However, to save heap allocations, it makes sense to retain it in the `Writer`
between functions. But the `Writer` field should have a different name, to
ensure people don't use it by accident.

Previously this was a bitcast

And remove the ability to read gl_Position in fragment.

Fixes issues with indexing vectors.