Not all Enums have a group
(Note: This is from master branch, which I assume is glad-1.)  
I'm currently trying to write a C++ generator for Glad (more in line with normal C++ ways of writing things, such as using `enum class name : underlying type {}` rather than many, many `#define`s).  

One thing I noticed while modifying the `CPPGenerator(Generator)` (for the most part, a direct copy of the C generator), in the `write_enums` method is that some enums don't have a group property even though it seems as if they should.  
For example: `GL_POINTS` is an entry inside of the group `PrimitiveType`, but it's group value is not set (`None`), and thus I can't properly get it's group and generate the enumeration.  
I currently build with: `python3 -m glad --out-path=buildcpp --spec=gl --api="gl=,gles1=,gles2=" --generator=cpp`  
To show this, with the normal C generator:
Modify the normal **C** generator `write_enums` method to have:
```python
if enum.name == "GL_POINTS":
    print('{} - {}'.format(enum.name, enum.group))
```  
Inside the for loop, and when ran it will print out:
```
GL_POINTS - None
GL_POINTS - None
GL_POINTS - None
```
When ran with `python3 -m glad --out-path=buildc --spec=gl --api="gl=,gles1=,gles2=" --generator=c`  
Is there any obvious reason for this that I am missing, or ways to make it work as expected?
