Flickering / glow backend not clearing buffer

[Hperigo]

Describe the bug
seems that egui frame doesn't clear properly using glow backend on a mac

To Reproduce
Steps to reproduce the behavior:

1. go to default example for the glow backend
2. add a global clear color variable
3. create a ui.color_edit_button_rgb(&mut clear_color);
4. move the color panel

Expected behavior
everything should be clear

Screenshots
https://user-images.githubusercontent.com/3333460/142732354-014755b9-9241-4ded-a673-138c8bb5622c.mov

Desktop (please complete the following information):

• OS: MacOSX
• Browser
• Version

Additional context
Only tested with glow backend in a Mac, so I'm not sure if this happens in other platforms

[20iahazban]

after digging deeper, i understand my comment does not belongs here but instead of delete my comment you could reference #838