Bug fix: framebuffer clear when using glow with multi-viewports

[emilk]

I broke this in #3665

For some reason, when using multiple viewports, the first "clear" doesn't take.

I don't have time to investigate further, so I am adding a hack here which:

• For single-viewport apps, the clear is done before App::update so user can paint there
• For multi-viewport apps, the clear is done after App::update, so that it works

This means painting in App::update won't work with multi-viewports.

Ideally, users should use paint callbacks instead of painting directly in App::update anyways.