glsl: Implement the required built-in functions when OES_shader_image_atomic is enabled.

This is basically just the same atomic functions exposed by
ARB_shader_image_load_store, with one exception:

    "highp float imageAtomicExchange(
         coherent IMAGE_PARAMS,
         float data);"

There's no float atomic exchange overload in the original
ARB_shader_image_load_store or GL 4.2, so this seems like new
functionality that requires specific back-end support and a separate
availability condition in the built-in function generator.

v2: Move image availability predicate logic into a separate static
    function for clarity.  Had to pull out the image_function_flags
    enum from the builtin_builder class for that to be possible.

Reviewed-by: Ilia Mirkin <[email protected]>

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