For libANGLE to work with our shaders, we must use "300 es" for
the #version directive in our shaders, as well as using the non-legacy/
non-GLES codepath in the shaders. In order to check whether we are
using the GLSL 300 es shaders, we check whether we are using a GLES 3.0+
context. As a result, make ->glsl_version a const char* and make sure
the existing shader version macros are defined apprpriately, and add a
new macro for the "300 es" shader version string.
This will allow the gtk4 programs to run under Windows using EGL via
libANGLE. Some of the GL demos won't work for now, but at least this
makes things a lot better for using GL-accelerated graphics under Windows
for those that want to or need to use libANGLE (such as those with
graphics drivers that aren't capable of our Desktop (W)GL requirements in
GTK.
On Windows with nVidia drivers at least, when we create a legacy context
via wglCreateContext(), we may still get a (W)GL 4.x context. Allow
such contexts to also use GLSL version 130 instead of 110, so that
things do continue to work.
