I broke this in https://github.com/emilk/egui/pull/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.
8503a85113