Fix buggy text withviewports on monitors with different scales (#3666)

* Closes https://github.com/emilk/egui/issues/3664

Bonus: optimize color conversions and font atlas upload, especially in
debug builds.

crates/eframe/src/native/glow_integration.rs

@@ -65,6 +65,8 @@ struct GlowWinitRunning {
 
     // These needs to be shared with the immediate viewport renderer, hence the Rc/Arc/RefCells:
     glutin: Rc<RefCell<GlutinWindowContext>>,
+
+    // NOTE: one painter shared by all viewports.
     painter: Rc<RefCell<egui_glow::Painter>>,
 }