ui/app: add Lock/Unlock to Context for macOS

Without locking, asynchronous OpenGL rendering crashes on macOS.

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

ui/app/gl_ios.go

@@ -83,6 +83,10 @@ func (c *context) Present() error {
 	return nil
 }
 
+func (c *context) Lock() {}
+
+func (c *context) Unlock() {}
+
 func (c *context) MakeCurrent() error {
 	if C.gio_makeCurrent(c.ctx) == 0 {
 		C.CFRelease(c.ctx)