glsl-in: support for storage image types

[JCapucho]

Adds support for the gimageDim family of types and the imageSize, imageLoad and imageStore functions.

Missing

• support for image types as function arguments (impossible see Format layout qualifiers on image function parameters? KhronosGroup/GLSL#57, writeonly images should work but see the following points)
• tests

Unresolved

• glsl supports multisampled storage images, naga doesn't, how do we handle it?
• glsl allows for writeonly storage images to not have their format specified, naga doesn't like it, once again how do we handle it?
• there are some texture formats to which I don't know the proper naga equivalent
• glsl allows for cube images which naga doesn't allow in imageLoad/Store, how do we handle it?

Closes #1456

[kvark]

glsl supports multisampled storage images, naga doesn't, how do we handle it

let's support it in the IR, but have a capability in the validator to check for it. Relevant SPIR-V capability - https://www.khronos.org/registry/vulkan/specs/1.2/html/chap31.html#features-shaderStorageImageMultisample

glsl allows for writeonly storage images to not have their format specified, naga doesn't like it, once again how do we handle it?

Not sure what to do with it. I suggest just not supporting this in Naga, at least for now.
SPIR-V has capabilities for either reads or writes to be unspecified with formats, but that would complicate our IR and backend logic possibly unnecessarily.

there are some texture formats to which I don't know the proper naga equivalent

We'll probably just need to add them.

glsl allows for cube images which naga doesn't allow in imageLoad/Store, how do we handle it?

This doesn't appear useful in modern APIs, since one would just create a 2D view instead. It looks like a GLSL artifact, so we shouldn't try supporting it.

[kvark]

Suggestions are optional, please land when you feel ready.

[JCapucho]

I've applied your suggestions, I'll merge this as an initial implementation and work on the missing bits that require some IR changes later when I have more time.