wflinfo: Use glGetStringi(GL_EXTENSION, i) for all contexts >= 3.0

Pre-patch, wflinfo used glGetStringi only for desktop OpenGL >= 3.0.
Unlike the OpenGL Core Profile, OpenGL ES 3.0 also supports
glGetString(GL_EXTENSIONS) for backwards compatibility with OpenGL ES
2.0. However, let's use the forward-looking API when it's available.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>