gamma_correction.nim(51, 44) template/generic instantiation from here

`..\..\src\easygl.nim(397, 30) Error: type mismatch: got <type GLfloat> but expected 'GLenum = distinct uint32'`

I tried to change to
```
when T is float32:
        discard vertexType.GL_FLOAT
```

it compiles, but fails with invalid enum as expected