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-wgpu/src/winit.rs

@@ -74,6 +74,8 @@ impl BufferPadding {
 /// Everything you need to paint egui with [`wgpu`] on [`winit`].
 ///
 /// Alternatively you can use [`crate::renderer`] directly.
+///
+/// NOTE: all egui viewports share the same painter.
 pub struct Painter {
     configuration: WgpuConfiguration,
     msaa_samples: u32,