gl: Use older GLSL shaders with legacy contexts
gl: Use older GLSL shaders with legacy contexts

If we're using modern GLSL, then we should stop using deprecated
modifiers, like 'varying' and 'attribute', as well as deprecated global
variables, like 'gl_FragColor'.

On the other hand, with legacy contexts we should be using older GLSL
shaders, to maximize compatibility.

https://bugzilla.gnome.org/show_bug.cgi?id=756142

alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita