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.

Allow vector sources and scalar selector
Do implicit conversions