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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 <imirkin@alum.mit.edu>