purpose of unsupported_extension class and context extensions

The `unsupported_extension` exception is currently only thrown by `opengl_create_shared_context()`. Maybe one could also throw it when the constructor of a general `context` fails, if there is an unsupported extension specified by `const cl_context_properites *properties`.

Maybe there also should be a way to construct contexts with extensions without having to create `0`-terminated lists by hand (maybe a vector class where the user can add extensions).
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