Store GLmetadata enums in String[] array

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Currently, all enums in all enum groups are simply stored in a HashMap, which is wasteful regarding space and runtime, since the enum values are rather densely distributed. Therefore, space requirements and lookup time can be improved by simply allocating a contiguous associative String[] array holding the names of all GLenum values in the effective interval min < max where min and max are the lower and upper bounds of assigned GLenum values.

On-demand loading of enum values into that array can still be used as is now, by lazily populating the respective needed array elements.