gpu: support forced OpenGL ES mode, for ANGLE on macOS

macOS is the only platform where desktop OpenGL is used. To support
foreign ANGLE contexts add a setting to override Gio's selection of
OpenGL implementation.

The bulk of this change is making all function pointers per-context
instead of global, and loading the OpenGL library dynamically. As a side
effect we're closer to Gio tolerating a platform without any OpenGL
implementation. For example, Apple has deprecated OpenGL and OpenGL ES
on its platforms and may remove them in the future.

Note that as a side-effect of this change, Gio needs Go 1.16 or newer to
run on iOS.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/wm/gl_ios.go

@@ -47,7 +47,7 @@ func newContext(w *window) (*context, error) {
 		return nil, fmt.Errorf("failed to create EAGLContext")
 	}
 	api := contextAPI()
-	f, err := gl.NewFunctions(api.Context)
+	f, err := gl.NewFunctions(api.Context, api.ES)
 	if err != nil {
 		return nil, err
 	}