Fixes around atomic operations (#652)

* Fixes around atomic operations

Work on #651

The existing handling of atomic operations had a few issues:

* The HLSL atomic functions (`Interlocked*`) didn't have mappings to GLSL

* Atomic operations on images weren't supported at all because the subscript operation on `RWTexture*` types didn't provide a `ref` acessor

* The HLSL atomic functions were only providing the overloads that return the previous value through an `out` parameter, and not the ones that ignore the previous value.

This change fixes these issues with the following changes:

* `RWTexture*` types now have a `ref` accessor on their subscript operation which maps to a new `imageSubscript` operation in the IR. By default this translates back to `tex[idx]` in output HLSL, but it makes a custom mapping possible for GLSL

* The `Interlocked*` function definitions were expanded to include the overloads without the `out` parameter

* GLSL translations were added for the `Interlocked*` functions. These mappings use some new customization points in the intrinsic operation emit logic to support outputting calls to either `atomic*` or `imageAtomic*` as required, and to expand an argument that is a subscript into an image as multiple arguments.

This whole approach is quite hacky, and it doesn't seem like the approach we should take in the long run.

* Fix: typo in InterlockedAnd lowering

One of the cases of `InterlockedAnd` was lowering to `atomicAnd` with a `$0` where we wanted the `$A` substitution to handle the possibility of an image.

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