Handle GLbitmask/GLbitfield correctly

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Here is what I think should be the correct way of handling those. The steps are being excercised for the example of "glBufferStorage" and its flags GLbitfield parameter:

1. for all /registry/enums elements that have a type="bitmask" attribute, the generator has to build a separate group with the enum values defined in that element (hint: the "group" attribute cannot be used for anything since it is hardly ever referenced anywhere)
2. for each /registry/feature/require (example GL_VERSION_4_4) and /registry/extensions/extension/require (example GL_ARB_buffer_storage) elements, we first have to search whether there is a <command> defined in it (example glBufferStorage) that takes at least one GLbitfield parameter. This information is found in the corresponding /registry/commands/command element.
3. If we found at least one such command, we have to match all enum elements in /registry/feature/require (example GL_VERSION_4_4) and /registry/extensions/extension/require (example GL_ARB_buffer_storage) with all previously created bitmask groups to see which of the enums are bitfield enums. For each enum found in a group A that group A is then considered a candidate group when looking up the name(s) of a GLbitfield argument by its value for a called command defined by this extension or core GL version.