Parsing extensions fails with Mesa and GLES2
Using glad 2.0.4 with the following [settings](http://glad.sh/#api=gl%3Acompatibility%3D3.3%2Cgles1%3Acommon%3D1.0%2Cgles2%3D2.0&generator=c&options=MERGE%2CALIAS%2CHEADER_ONLY).

Calling `gladLoadGLES2` with Mesa implementation returns 0 because GLES2 functions are resolved but extensions parsing failed.

This is due to the fact that Mesa returns GLES version 3.2 and `glad_gl_get_extensions` gets called with this 3.2 version.
`GLAD_GL_IS_SOME_NEW_VERSION` is defined because `GL_VERSION_3_0` is defined (as we generate it).
The version check occurs and ends up in the `version >= 3` branch.
Sadly, the GLES2 loader code didn't try to initialize `glad_glGetStringi` (as it's not defined in GLES2) and the sanity check fails which makes loading extensions fail.

it seems to me that `glad_gl_get_extensions` should depend on the API calling it and if GLES2 is used, it should always use the basic `glad_glGetString(GL_EXTENSIONS)` call.

The same should occur for GLES1.

Thanks.