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/egui_glow/src/painter.rs

@@ -59,6 +59,8 @@ impl From<String> for PainterError {
 ///
 /// This struct must be destroyed with [`Painter::destroy`] before dropping, to ensure OpenGL
 /// objects have been properly deleted and are not leaked.
+///
+/// NOTE: all egui viewports share the same painter.
 pub struct Painter {
     gl: Rc<glow::Context>,