Improve text rendering in light mode (#7290)

This changes how we convert glyph coverage to alpha (and ultimately a
color), but only in light mode.

This is a bit of a hack, because it doesn't fix dark-on-light text in
_dark mode_ (if you have any), but for the common case this PR is a huge
improvement.

You can also tweak this yourself now using
`Visuals::text_alpha_from_coverage` or from the UI (bottom of the
image):


![image](https://github.com/user-attachments/assets/350210d4-c0bb-44b6-84cc-47c2e9d4b9f0)



## Before / After

![widget_gallery_light_x1](https://github.com/user-attachments/assets/21f5a2a0-6b4e-4985-b17f-cd1c7cc01b46)
![widget_gallery_light_x1](https://github.com/user-attachments/assets/5dfec04a-c81c-43ef-8d86-fc48ef7958f1)


## Black text Before/after
If you think the text above looks too weak, it's only because of the
default text color. Here's how it looks like with perfectly `#000000`
black text:


![image](https://github.com/user-attachments/assets/56a4a4f3-c431-4991-b941-a566a4ae94ed)
![Screenshot 2025-07-02 at 13 59
30](https://github.com/user-attachments/assets/df5a91ad-0bb8-4a0f-81a2-50852e7556c1)

crates/egui-wgpu/src/renderer.rs

@@ -571,7 +571,11 @@ impl Renderer {
                     "Mismatch between texture size and texel count"
                 );
                 profiling::scope!("font -> sRGBA");
-                Cow::Owned(image.srgba_pixels(None).collect::<Vec<epaint::Color32>>())
+                Cow::Owned(
+                    image
+                        .srgba_pixels(Default::default())
+                        .collect::<Vec<epaint::Color32>>(),
+                )
             }
         };
         let data_bytes: &[u8] = bytemuck::cast_slice(data_color32.as_slice());