cefclient: port to GTK3

This patch ports cefclient demo from GTK2 to GTK3. The reason behind that is Wayland support that I add to CEF. GTK2 doesn't have Wayland support and it's impossible to run the demo app with that backend. Thus, update the app so that it works with GTK3.

Please note that updating to GTK3 resulted in dropping gtkglext that used OpenGL2. Modern systems use at least OpenGL3 and onwards. This resulted in OSRRenderer failing as some of the APIs 1) are deprectated 2) and it's also required to write own shaders and utilize vao, vbo, and maybe ebo.

Thus, in order to run the OSR case with cefclient, one must force the app to use OpenGL <=3.1. For example, MESA_GL_VERSION_OVERRIDE=3.1