[contrib.glfw3] Fixes GLFW_CONTEXT_VERSION_MAJOR ignored (#23401)

This is a follow up to the
[ticket](#23372)
discussion.

When users are using my port, the window hint
`GLFW_CONTEXT_VERSION_MAJOR` is ignored due to how Emscripten behaves
internally.

If the port sets the linker flag `MAX_WEBGL_VERSION=2`, then for a user
of the library, which interact throught the GLFW API, it transparently
works without asking the user to understand the internals of Emscripten
and modify their build script.

tools/ports/contrib/glfw3.py

@@ -88,6 +88,7 @@ def linker_setup(ports, settings):
   root_path = os.path.join(ports.get_dir(), port_name)
   source_js_path = os.path.join(root_path, 'src', 'js', 'lib_emscripten_glfw3.js')
   settings.JS_LIBRARIES += [source_js_path]
+  settings.MAX_WEBGL_VERSION = 2 # for GLFW_CONTEXT_VERSION_MAJOR to work
 
 
 # Using contrib.glfw3 to avoid installing headers into top level include path