glad2: allow custom symbol prefix (instead of glad/Glad/GLAD)
Two reasons for this change:
 * Prevent symbol collisions when linking statically.
 * Better integrate into the host project.

One reason against:
 * Templates become very messy. Example (note the `GLAD`/`Glad`/`glad` prefix vars):
```c
{{ GLAD }}_API_CALL int {{ glad }}Load{{ api|api }}{{ 'Context' if options.mx }}UserPtr({{ template_utils.context_arg(',') }} {{ GLAD }}userptrloadfunc load, void *userptr);
{{ GLAD }}_API_CALL int {{ glad }}Load{{ api|api }}{{ 'Context' if options.mx }}({{ template_utils.context_arg(',') }} {{ GLAD }}loadfunc load);
```
If you think it's worthwhile, I'll prepare a PR.