gskglcompiler.c: Force GLSL version 300 es as needed

gskglcompiler.c: Force GLSL version 300 es as needed

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.