Add support for `image` memory qualifiers about slang

[tangent-vector]

This will be easy enough to add. If you only need them to be parsed, and don't need reflection, you should be able to do it yourself as a workaround until I can get to it.

In source/slang/slang-stdlib.cpp, near the end, you will find these lines:

        sb << "__modifier(SimpleModifier)           flat;\n";
        sb << "__modifier(SimpleModifier)           highp;\n";

You should be able to do the same thing to add as many qualifiers as you like, that Slang will parse, ignore, and then spit back out. E.g.:

        sb << "__modifier(SimpleModifier)           coherent;\n";
        sb << "__modifier(SimpleModifier)           volatile;\n";
...

I'll try to do a more thorough survey of GLSL qualifiers soon.